Greetings!

I currently booked SFO-LHR roundtrip on business, I'm considering book a separated LHR-NCL (economy only) roundtrip to make it actually SFO-LHR-NCL/NCL-LHR/LHR-SFO.

My questions are:

1. I guess I'll need to process passport control at LHR, do I have to recheck my bag at LHR as well (as in the USA), or I can tag them all the way to NCL at SFO (even on separated bookings)?

2. How many time would it generally take for the passport control/custom process at LHR, as a non-UK non-EU citizen? My SFO-LHR is expected to land at 14:45, the 2 LHR-NCL flights after that would be 18:00 and 20:45, I'm not sure if the 18:00 one would be safe enough (since they are separated booking, I guess BA wouldn't change me to the later one for free if I booked and missed the 18:00 one?)

Thanks!