Jacko,
Per some quick googling, the RAF perferred the US made J-79 engine for the F4 but had to convert to the Spey for commonality with the FAA F-4's. The Spey allowed better acceleration, climb, and range but made for a much slower airspeeds.
The British Phantoms could be distinguished from their American cousins by their wider and shorter engine exhausts. Beginning in 1975, RAF Phantoms were fitted with a distinctive rectangular antenna for a Marconi ARI 18228 RWR, attached to the top of the vertical tailplane.
The changes ordered by the British government led to a considerable increase in the cost of the aircraft, and worse, although the Spey was more powerful than the J79 and had greater fuel economy, the British would probably have been better off to have stayed with the J79. The match between an engine and an airframe can be surprisingly tricky, and the Spey-powered Phantom turned out to be a textbook example of the troubles that can arise when trying to mate an engine to an aircraft that wasn't designed for it from the outset.
The British Phantoms required substantial airframe modifications, with the rear fuselage width increased to accommodate the Speys, and 20% larger intake ducts to provide the greater airflow required by the Rolls-Royce engines. Even then, the Speys were temperature-redlined to Mach 1.9, due to cost-reduction measures imposed on Rolls-Royce that dictated use of cheaper metals. Between the airframe changes, which undermined the Phantom's "area ruling", and the limitations of the engines, the British Phantoms were sometimes described, most likely by a certain famously caustic senior British aviation writer, as the "most powerful, most expensive, and slowest Phantoms in the world."
However, although the 1960s were the great age of British weapons procurement fiascos, the selection of the Spey was by no means a case of excessive nationalism at work, and the problems seem to have been sometimes exaggerated. Since British carriers were smaller than American carriers, FAA Phantoms had to have more power to get off the decks, both in terms of drive for the BLC system and greater thrust. In fact, the Spey-powered F-4s had superior acceleration and rate of climb to J79-powered F-4s, outpacing US Navy Phantoms during their stint on the USS SARATOGA until their American cousins could catch up and then fly past them.
The British Phantoms also had better low-level performance, an absolute benefit for RAF strike pilots who normally flew above the treetops, and enjoyed a 10% greater radius of action. Ground-attack pilots particularly enjoyed knowing that they could take on adversary fighters at level or superior odds. The fact that the Phantom could carry four AAMs on the fuselage recesses without interfering with carriage of attack munitions was a significant advantage.
As a strong plus, from 1979 on the British Phantoms carried the UK-built "Sky Flash" AAM in preference to the American-made Sparrow. Fit of the Sky Flash to the Phantom was simple, because the missile was based on the Sparrow airframe, with British-built seeker. The Sky Flash was much superior to US-built Sparrows of the time, spurring the Americans to catch up with improved Sparrow variants of their own.
The RAF's initial misgivings about the type gradually grew into admiration and affection, one RAF pilot concluding: "All in all, the Phantom was an excellent aircraft." It was retired with regrets. A number of "Yanks", such as the C-47 Dakota and the P-51 Mustang, have served with distinction wearing RAF roundels, just as the British Canberra and Harrier have served with distinction wearing the red-white-and-blue in return. The Phantom was one of the significant American eagles in British colors, proving capable enough to overcome the nationalism that sometimes puts the two nations at odds.
A very good discussion of the history of the F4 in UK service.
http://www.thunder-and-lightnings.co...m/history.html