I think the RN order was 120 aircraft, it came as a bit of a surprise that the first to arrive in the UK, were in RAF colours.
So I'm not sure that it was 20 only for the F4K
AIUI
the initial order was 140 new build FG.1 for the RN
with 20 of these diverted to the RAF when the upgrade to Ark Royal cancelled
(and OTL probably others later.
IIRC 2 whole squadrons of the RAF used basically unmodified FG.1 until they converted to Tornado ADV)
OTL this was later cut to 50 with only 24 + spares planned for deployment afloat, leaving some on land.
iTTL at least that many will remain on order.
PLUS
150 new build FGR.2 ordered for the RAF when the F-111K was cancelled,
however 32 of these themselves cancelled
Aside:
15 F-4J from US mothballs were refurbished to be a F.3 BUT after the Falklands so are not relevant
Losses from accidents etc. for all types were high so not all of these would be available iTTL at the TL date.
In any case, as others have said, a FGR.2 could NOT be swiftly converted to carrier operation
Any FG.1 transferred to the RAF are probably easier but I doubt many would be released immediately.
However any other FG.1 being used from land e.g. in training or testing
plus any still in RN service but in store or mothballed when their carrier was stood down would be easy.
IMHO the FAA would have started a "rush" refit on any such available FG.1 that could be made flyable
as soon as the Task Force was given the go ahead if not sooner.
Not all could be reactivated to full mission capability but a handful certainly could.
Arrangements to fly replacements south would have been in place long before Eagle got into harms way.