Flying into FCO from JFK with a layover in LHR in March. RT tickets back to JFK
We are splitting a 9 day trip between Rome and Croatia.

Ideally I would like to fly into Rome and then continue onto SUP that same day instead of having to fly out of Rome after a couple days and then back to Rome to fly home.

Our flight lands at 4.25 pm in FCO. Having the lay over in LHR will we clear European immigration in the UK or FCO ?
There is a Alitalia flight from FCO-SPU at 6 pm.
Is 1.5 hours enough time to make that connection,

This will be two separate tickets, as I'm flying BA into FCO.

Thank you