Although there were minor differences between the two types of Phantom built for the UK, there were many significant ones between the British Phantoms and those built for the United States.
The most obvious was the substitution of the Rolls-Royce Spey turbofan for the General Electric J79 turbojet.
The Spey was shorter but wider than the J79, which meant that the British Phantoms' intakes had to be redesigned for a higher airflow, making them 20% larger (with a consequent increase in drag), while the fuselage was widened by 152 mm (6.0 in).
The position of the afterburner also meant that the rear of the fuselage had to be made deeper.
Auxiliary intake doors were fitted on the rear fuselage.

Performance estimates of the British Phantom compared to its American equivalent indicated that the former had a 30% shorter take-off distance, 20% faster climb to altitude, higher top speed and longer range.
The Spey was more efficient at lower altitudes, and had better acceleration at low speed, giving British Phantoms better range and acceleration, which was shown during the deployment of 892 NAS to the Mediterranean aboard USS Saratoga in 1969, when the F-4K was repeatedly quicker off the deck than the F-4J used by the Americans.
It was less efficient at higher altitudes, the British Phantoms lacking speed compared to J79-powered versions owing to the increased drag of the re-designed fuselage.
This discrepancy became apparent when the F-4J was obtained by the UK in 1984; it was regarded as being the best of the three variants to serve in the RAF.

The small size of the aircraft carriers Eagle and Ark Royal, from which the Royal Navy's Phantoms were intended to operate, compared to the US Navy carriers of the period, meant that the F-4K version required significant structural changes compared to the F-4J, from which it was descended, and which performed a similar role.
As well as the folding nose radome to allow for storage in the smaller hangars of the British ships, it had to have a significantly strengthened undercarriage to account for the higher landing weight (British policy was to bring back unused ordnance).
The F-4J featured a nosewheel oleo that extended by 20 inches (51 cm) to provide the correct attitude for launch from American catapults.
The F-4K's nosewheel oleo extended by 40 inches (100 cm) to increase the take-off attitude (the extension of the nosewheel put the Phantom at a 9° attitude) due to the shorter and less powerful British catapults.
It was also fitted with drooping ailerons, enlarged leading edge flaps and a slotted tailplane, and increased flap and leading edge blowing, all to improve the lift and handling characteristics of operation from the much smaller carriers of the Royal Navy.

As the Phantom continued in service, other changes were made, most notably the Marconi ARI.18228 Radar Warning Receiver (RWR) fitted on top of the vertical stabiliser, exclusively on British models.
The RWR was fitted in the mid-1970s to the FG.1 and FGR.2 Phantoms, but not to the F.3.
From 1978, the Skyflash AAM, derived from the AIM-7 Sparrow, began to be delivered to RAF Phantom units, and was used concurrently with the Sparrow; all three UK Phantom variants were eventually fitted to operate the Skyflash.
Although there were minor differences between the two types of Phantom built for the UK, there were many significant ones between the British Phantoms and those built for the United States. The most obvious was the substitution of the Rolls-Royce Spey turbofan for the General Electric J79 turbojet. The Spey was shorter but wider than the J79, which meant that the British Phantoms' intakes had to be redesigned for a higher airflow, making them 20% larger (with a consequent increase in drag), while the fuselage was widened by 152 mm (6.0 in). The position of the afterburner also meant that the rear of the fuselage had to be made deeper. Auxiliary intake doors were fitted on the rear fuselage.
Performance estimates of the British Phantom compared to its American equivalent indicated that the former had a 30% shorter take-off distance, 20% faster climb to altitude, higher top speed and longer range. The Spey was more efficient at lower altitudes, and had better acceleration at low speed, giving British Phantoms better range and acceleration, which was shown during the deployment of 892 NAS to the Mediterranean aboard USS Saratoga in 1969, when the F-4K was repeatedly quicker off the deck than the F-4J used by the Americans. It was less efficient at higher altitudes, the British Phantoms lacking speed compared to J79-powered versions owing to the increased drag of the re-designed fuselage. This discrepancy became apparent when the F-4J was obtained by the UK in 1984; it was regarded as being the best of the three variants to serve in the RAF.
The small size of the aircraft carriers Eagle and Ark Royal, from which the Royal Navy's Phantoms were intended to operate, compared to the US Navy carriers of the period, meant that the F-4K version required significant structural changes compared to the F-4J, from which it was descended, and which performed a similar role. As well as the folding nose radome to allow for storage in the smaller hangars of the British ships, it had to have a significantly strengthened undercarriage to account for the higher landing weight (British policy was to bring back unused ordnance). The F-4J featured a nosewheel oleo that extended by 20 inches (51 cm) to provide the correct attitude for launch from American catapults. The F-4K's nosewheel oleo extended by 40 inches (100 cm) to increase the take-off attitude (the extension of the nosewheel put the Phantom at a 9° attitude) due to the shorter and less powerful British catapults. It was also fitted with drooping ailerons, enlarged leading edge flaps and a slotted tailplane, and increased flap and leading edge blowing, all to improve the lift and handling characteristics of operation from the much smaller carriers of the Royal Navy.
As the Phantom continued in service, other changes were made, most notably the Marconi ARI.18228 Radar Warning Receiver (RWR) fitted on top of the vertical stabiliser, exclusively on British models. The RWR was fitted in the mid-1970s to the FG.1 and FGR.2 Phantoms, but not to the F.3. From 1978, the Skyflash AAM, derived from the AIM-7 Sparrow, began to be delivered to RAF Phantom units, and was used concurrently with the Sparrow; all three UK Phantom variants were eventually fitted to operate the Skyflash.
McDonnell Douglas Phantom in UK service