I'm booking a 3-leg trip: USA-FRA-LHR-ORK. I'm unclear on where in Europe I'll need to clear immigration (more than once?), and where (in transit) I'll need to go through additional security. I'm trying to figure out where I need the extra time - FRA or LHR.

I'll have about 7 hours total, and my preference is the extra time in FRA as I'll have lounge access there (LH C), but not in LHR, I think (as arrivals lounges will be closed and my continuing flight is in another terminal, in economy, on a different airline, EI).

I could do:
5h45m FRA
2h15m LHR
or
4h45m FRA
3h15m LHR
or
6:45 FRA
1:15 LHR

Any advice would be much appreciated!