Is 90 mins too tight to land (on time???) go through immigration, change terminal and board other plane? I'm thinking it is, but the alternative is to allow 4 hours, but have to get to La Guardia instead. Also, am not too mobile at the moment (recovering from broken leg, although 4 months on) so rushing won't be an option.
It could easily take you at least 90 minutes standing in line waiting to be processed at immigration!. There are a lot of delays at present at some US airports with regards to immigration re processing pax as they arrive due to overall reduced staffing levels (due to budget cuts). Miami like JFK is similarly affected.
Also if you need to change terminals at JFK it may well be that you will end up landside and thus will need to go through security again.
I would go for the 4 hour stopover instead; 90 minutes is far too tight a timescale in any event and you should have never been offered that choice. Also you need to bear in mind that the clock starts ticking as soon as the plane lands from the UK.
Jfk is massive and can take ages to navigate even on a good day. I've stood in passport control line for almost an hour on a few occasions ( and that's with a US passport!) so I'd def go with the 4 hour option.