Originally Posted by
euslaner
I had a connection from UA last year to BA, IAD-LHR-MAN. Separate PNRs, but UA assured me that they would interline my bag. Got to MAN, no bag. BA told me that they do not interline with *A carriers. On the return, they refused to check it through all the way back to IAD. Had to reclaim it.
Even worse, when I boarded the BA flight LHR-MAN, I knew my bag wasn't on board. A GA pulled me aside as I was boarding and told me that my ticket showed that I had checked a bag, but they didn't know where it was. I showed them the baggage tag I got at IAD and they just shrugged. They claimed that UA never got the bag to them. But when I arrived at LHR, I checked with UA and they assured me that my bag had been transferred to BA.
I would not take any assurances from BA as worth more than the paper they aren't written on.
The UK requires passenger-baggage reconcilliation for all flights (the US doesn't this for internal flights). So BA would have had no record that your bag was meant to be on their flight if you had separate tickets, and thus would have refused to take it on the outbound. Furthermore, BA has a policy of not interlining with non-Oneworld carriers on separate tickets, which, as you say, was have been the problem on the return.
As the OP's itinerary is all on one PNR, there
should be no problem. I would be extremely surprised if United and BA didn't have an interline agreement.
Lastly, the OP will go through immigration at Heathrow, but because their bag is checked all the way through, needs to go though customs at Manchester. In the UK however, customs is simply a matter of not doing anything at all, unless you have something to declare.