you can check your bags all the bags thru to LHR , no problem.

Look for the 1st class counter , most Right counter of COs

as for arrival time Id say 60 mins min but Im not sure why not give CO a call and ask

theres a UA RCC (lounge at SNA) I think being that you are on a Intl BF tkt that you might be able to get in, theres no PC @ SNA, so ask at checkin