After its recent $2.5-billion buyout of Jaguar Cars, Ford Motor Co. could be excused for declaring July 4 a factory holiday here and Americanizing brochures that refer to bonnets, fascia colours and spare tyres in boots.

But to change one whit or the wildcat in Jaguar's new, elegant, more powerful but less expensive XJ6 would be an unforgivable breach of Anglo-American relations.

Correction. Ford might like to shine one small inspection light on Jaguar's electrical system--and in particular the company's lasting dedication to the flickering equipment manufactured by British-based Lucas.

Jaguar's electricals have been glitch-rich for years. There was some improvement when Sir John Egan assumed command of Jaguar in 1980. But the company has yet to insulate all electrical problems, and flickers or blackout should not be part of any car costing more than a year's rent at the Savoy.

On our test vehicle, the windshield wipers stopped wherever the poor dears felt weary. Central door locking was iffy with a key-chain remote control that kept flopping apart and dumping its battery. Definitely not cricket for a car built by a company with a knight for a CEO.

Beyond that, the new and vastly improved XJ6 for 1990 remains faithful to a bloodline that since the SS100 of the '30s and through the Jaguar XK120s and Mk VIIs and E-Types of the '50s and '60s has evolved into a wonderful combination of luxuriant muscle.

In one of its motoring moods, the XJ6 is all leather and wood smells and purring graciousness fit for the royal family that, indeed, garages its Jaguars at Buckingham Palace. Up Brompton Road to Harrods or down Jermyn Street to Fortnum & Mason, the XJ6 satisfies our senses of place and personal pride.

Yet in a more raucous mode, it will blast through motorway traffic with the howling verve expected of any direct descendant of the C- and D-Type Jaguars that five times conquered Le Mans.

In terms of controlled cruising, power-adjusted seating and lacquered walnut per square inch of door and dash trim, the XJ6 wears no more options than any other luxury performance car. Its straight-six, 4-liter engine is no ground pounder alongside the mighty V8s of Chevrolet, Ford and the heftier Mercedes.

But Jaguar has been doing it so much longer than Lexus and Infiniti, and there's security in that. While Mercedes or BMW opt for quantum leaps and model ranges aimed at market niches from basement to Beverly Hills, Jaguar prefers perpetual refinement of the proven. Its mechanical foundation, its straight-six engine, for example, hasn't changed that much since it was powering the XK sports car series.

To some, that's decadent. To others, it is the creation of tradition. The undeniable end result is a car of familiarity and substance, offering that sense of enduring quality the British do so very well.

Last year, however, there were quiet fears that Jag uar would be forced into reducing its polish and prestige. The company's U.S. sales had slumped as the pound strengthened and the dollar weakened. Competition in the luxury performance car market increased. Worse, Lexus and Infiniti were selling for $5,000 less than the least expensive Jaguar.

Jaguar had only one hand to play. In October, the company announced a $4,300 price cut for the XJ6. Some little luxuries would be left out. But the famed, pure, sweet straight-six engine would grow from 3.6 liters to 4 liters with an increase in horsepower from 195 to 223.

Doubts rose. But "downgrading" and "entry level" and "down market" were phrases used by others in discussing the XJ6.

"We have simply adjusted the specifications," explained a Jaguar spokesman this week. "The new car retains all the traditional Jaguar features--the walnut trim, leather on the seats, full instrumentation--while remaining competitive across a broader (market) range."

"Lasty ar, we offered (in the United States) three models (of Jaguar) with a price range of $44,000 to $53,000. This year, we're simply offering four models ranging in price from $39,700 to $53,000. And now we're in a position to say to the potential Lexus buyer: 'Look, you can have a Jaguar for that (same price) with all that owning a Jaguar means.' "

Even at its smaller price, the 1990 XJ6 still comes with cruise control, alloy wheels, air conditioning, six-speaker sound system, anti-lock braking, leather-faced seats, air conditioning, trip computer, power windows, seats, side mirrors and central locking--but no air bag.

Missing is last year's power sunroof; a rear load-leveling system; matched, burled and inlaid walnut trim (this year's wood is plain), and some bright work has been muted to black. But for buyers who can't live without any of that, there is a fully optioned XJ6 that costs $43,000 and is now known as the Sovereign.

Our test car was maroon and wearing a pair of tacky U. K. basics--plastic wheel covers and fabric seats--that we pray will never be exported to this country by FoMoJag.

And in keeping with Britain's recent and thoroughly belated education in the horrors of hydrocarbons, nitrogen dioxides and the full range of automotive smog, this XJ6 came equipped with a catalytic converter (an option in the United Kingdom), which trimmed its performance to just about U. S. standards.

There is no real nippiness in that performance, no snappy steering or popping of clutches associated with parking lot slaloms and country lane duels with errant Peugeot Mi16s.

With a width of 6 1/2 feet (identical to a Rolls-Royce), the XJ6 is a little too beamy to hustle. With an avoirdupois of two tons (and no manual transmission to guarantee a polished balancing act), it certainly is a mite too heavy to be slinging around wet macadam.

If pushed in tight corners, of course, the Jaguar will deliver, and with as much response and delicacy as any other middleweight. Its metier, however, is rapid touring amid the quiet comfort of a factory-made car that carries the heft of a hand-built one.

And it is rapid touring. From breakfast until teatime, the Jaguar will cruise happily at 110 m.p.h. We became a little zealous on one stretch of the broad and dry M4 to London and were looking at 130 m.p.h. before coming upon the nuisance of a trio of lane-changing lorries.

The acceleration of the XJ6--from rest or in passing mode--is simply superb for a car of its bulk. Should that not be enough, Jaguar's automatic transmission has been equipped with a handy "sport-normal" control that in "sport" setting will hold each gear closer to its performance peak.

Twice (once for the worst case scenario of a woman wheeling baby and perambulator into a crosswalk) we got into the anti-lock braking system. Twice we went against the belts as the big cat stopped dead, absolutely str aight and without a suggestion of slither.

The steering was neither rigid nor fluffy, the suspension neither buckboard nor sponge cake. By no accident. Mechanical completeness comes only from years of breeding; a craftsman's tweak here, an engineer's fillet there and an ounce of pressure all around. Until a system has progressed from adequate for some, to optimum for most.

This mechanical harmony extends to the interior of the XJ6, where all dials are readable, handles are to hand and controls are not a symbology quiz. Night driving--thanks to a perfect combination of cabin and instrument illumination--is absolutely fumble-free.

Add the comfort of club-class seating and a driving ease that allows not only listening but also mental conducting of Vivaldi at 100 m.p.h., and you have a carriage that on London-to-Edinburgh runs could well put Virgin Airways out of business.

You also have a car that continues to live up to its advertising claim of the m dieval '50s. The reference was to the Jaguar Mk VII, and it was a three-word promise of "Grace. . .Space. . .Pace."

1990 Jaguar XJ6

The Good Increased power and sustained quality that costs less. Gracious looks, comfortable roominess. Quiet, fast, effortless ride for serious touring. Substance from pure heritage.

The Bad Unreliable electricals. Thirsty engine.

The Ugly Excuse me?

Cost Base and as tested $39,700.

Engine 4.0 liters, six cylinders, in-line, developing 223 h.p.

Performance 0-60, as tested, 8.6 seconds. Top speed, manufacturer's claim, 136 m.p.h. Fuel economy, EPA city-highway, 17-22 m.p.g.

Curb Weight 3,969 pounds.

Rebate Program None.