The Royal Navy Phantoms were handed over to the RAF when the FAA ceased operating conventional fixed-wing aircraft from carriers, following the change to the Sea Harrier.
Both the FG1 and FGR2 were in service with the RAF after this time, but issued to specific squadrons - the type wasn't mixed, and (normally) the sub-type squadrons would be based on the same airfield, for ease of maintenance, stores etc etc.
Externally, there is little difference between the two, apart from the extra torque link for the extending nose gear leg, different lamps on the nose gear door, catapult hooks beneath the wings, and slotted, or 'plain' tail plane. From memory, with the possible exception of the tail plane, the Hasegawa RN and RAF FG1 kits include the parts for the nose gear doors and the lamps, and 'blanks' for the catapult hook mounts, with the extra torque link being either fitted or omitted, using the nose gear leg as provided (the 'extended' leg is a separate, optional part).I'm not sure if the FGR2 kit includes the parts for the FG1 though, so it might be better to stick with a RAF version, from the squadrons that operated this type. Again, I can provide the info on this later today.
I can post more detailed info later today - I'm off to bed, as it's stupid o' clock here!
I originally built mine as a 43 Sqn FG1, ex RN, using the 'Ark Royal' issue of the kit, and then 'converted' it to a 56 Sqn FGR2 by removing the extra torque link, filling the apertures for the catapult hooks, changing the nose gear door, and filling in the 'slots' on the tail-planes, then just a change of serial number and markings.
Internally, of course, there were a number of differences, including the cockpits, but these can't be seen on a model, apart from some small cockpit details.
First of all, identify which squadron you want to portray, and the time period, and this will provide the type, and the colour scheme.
There was also one other squadron added to the Phantom fleet after the Falklands campaign, 74 'Tiger' Squadron, equipped with the F-4J(UK), ex - US Navy airframes, painted in the grey scheme, different to the RAF pattern, and called Flint Grey. When these became 'worn out', they were, in turn, replaced by 'British' Phantoms, from squadrons converting to the Tornado F3.
If you do decide to get another one, then look out for the Revell re-boxing of the FGR2, which has 56 Sqn decals, in the 'grey' scheme, as one of the options, and, although still not particularly cheap, it's not as daft a price as the Hasegawa kit. I got my Hasegawa example back in the late 1980's, and it was £25 even then - a lot of dosh for nearly 30 years ago !