Let's look at OTL Phantom customers, for a start...
Phantom Service with Royal Australian Air Force
Phantom Service with Egypt
Phantom Service with Germany's Luftwaffe
Phantom Service with Iran
Phantom Service with Greece
Phantom Service with Turkey
Phantom Service with Japan
Phantom Service with South Korea
Phantom Service with Spain
Phantom Service with Israel
Phantom Service with United Kingdom (d'oh !)
Israel is an interesting case. OTL before De Gaulle screwed the pooch in 1967,IDF/AF had little love for the USA (yes, it sounds completely mind-blowing writting that nowadays)
Circa 1966 they already knew the writting was on the wall with the French, and were looking for alternate source of combat aircraft.
Note that they had bought Meteors a long time before, and in the mid-50's were desperate to get supersonic anti-MiG-15 aircraft and even asked the Swedes for J-29 Tunans, and Canada for Canadair F-86s).
If Great Britain strongly hangs to the P.1121 in the 60's and beyond (instead of the OTL insanity between Sandys and Tornado: 15 years in hell, 1958-1973) I can see it as a strong alternative for IDF/AF long range strike Vautour replacement, 1965.
IDF/AF considered
- Super Vautour (Tsykklon / Cyclone: a huge upgrade with... Spey turbofans )
- Mirage F2
- Mirage IV
They finally went for Mirage V and then, once embargoed, for Nesher / Skyhawk / Phantoms.
I can really see P.1121 sneaking in there - provided the British government accept to help Israel, obviously. Note that they would screw both French and Americans, in the process.
Greece and Turkey got shitloads of US combat aircraft, all the way from F-84 to Phantoms, including F-5s, F-102s (!!) F-104s... and they thought very symmetrically. "Greece has bought second hand F-102s ! I want them, too." If GB manages to drop some P.1121s to one, the other one is a done deal.
By contrast - Spain, Franco and Gibraltar... FORGET IT !!!
Australia is a strong contender. Frack, on paper at least P.1121 could sneak into the shoes of (altogether !) Mirage interceptor 1961 and the Phantom / F-111 OTL mix, 1965-1975.
IF the Australian domino fells, chance are strong Canada might bought some P.1121 too. The irony would be insanely strong there - replacing the Arrow (facepalm), and OTL CF-101s and CF-104s. The P.1121 smashes the last two, performance-wise. For strike, and interception.
And then the fantastically dumb CF-5 procurement siliness could get through the window, too. More P.1121 there.
Japan and South Korea unfortunately are Uncle Sam jealously guarded playground, so no chance in hell.
And then there is Iran in the days of the Shah. OTL his first huge buyout spree related to F-5 - and he got a colossal number of them, quickly followed by Phantoms. All this after 1965 and before the 70's pornfest.
Would the Shah buy P.1121s ? Maybe. He was extremely ambitious right from his first F-5 order in the mid-60's. If GB manages to sell P.1121 instead, once again it brings together IRIAF OTL F-5 and F-4 orders. Which quickly mushroomed into giganormous numbers - from memory, some hundreds... Quick Google search brings 308 and 225 aircraft:
533 combat aircraft, for fuck sake.