I'm flying SFO-LHR on a Sunday on VS, arriving at 10:30 AM at Heathrow.

I booked a BMI flight, LHR to CDG for 13:10.

I figured I'd have plenty of time to reclaim any baggage, get through Customs and check-in for the BMI flight.

But reading about the immigration queues, I'm starting to wonder.

I believe the VS flight arrives (and departs) at Terminal 3 while the BMI flight arrives and departs from Terminal 1?

Yes, plenty of time. 2hr 35min.

Virgin and BMI have codeshare and baggage agreements, so when you check your bags at SFO they should be tagged for CDG. In fact, you should insist on it. You'll go through bag inspection (or none as the case may be) at CDG.

Thus on arrival at LHR you just go to the flight connections desk and transfer straight to T1. You'll have gobs of time.

OK the Virgin and BMI flights were booked entirely separately. But I will ask Virgin and BMI about that.

One thing I wonder though is that on smaller planes for intra-European flights, they have more baggage restrictions, so my carry-on for the Virgin flight might have to be checked in as well.

Or do they just take your bigger carry-ons at boarding time since they wouldn't fit on the smaller overhead compartments?

If they are booked on separate tickets then VS <i>ought</i> to agree to through-check them IF you present your onward ticket to them at the SFO counter. If not, then you will have to claim and recheck at LHR; you should still have enough time though.

Yes, carryon rules are very tight. BMI says 7 kg + computer for shorthaul cabin bag allowance.