U.K. and Ireland - Transferring WIHTHIN LHR T1 without immigration - possible?

Flying from FRA to SFO through LHR. LH for FRA-LHR and UA for LHR-SFO. Travelling with two Russian citizens that would normally require British visas. Connecting time at LHR is just under 3 hours. We do not intend to enter the UK, just to transfer at LHR within the same terminal. Is that possible or will they force them to go through immigration? Also, do you think LH may have any issues with issuing them boarding passes for that transfer? Thanks

As long as your bags are checked through, if you have checked baggage, just follow the purple connection signs. You will not clear immigration, but will have to clear security again before entering the departures lounge.

You should be fine. There's no immigration crossing even for terminal change. I just did it this weekend.

Delta offer free access to the TIMATIC database which is what checkin staff will look at if necessary to determine visa requirements.

http://www.delta.com/planning_reservations/plan_flight/international_travel_information/visa_passport_information/index.jsp

I reckon you'll be OK.

There won't be any issues. If you don't all get your LHR-SFO boarding passes in FRA, there's a UA desk just after the transfer security checkpoint which will print them for you. In fact, you might want to stop by there anyway, as they may need to swipe your passports or check the SecureFlight information.