Thanks again for all the helpful tips.
It's a good point about the positioning flight and what happens if my bags are delayed. I was thinking it's not the end of the world, as long as they reach me in the end. But if they can't complete the journey without me being present to claim and re-check them at PHL, then I'm in trouble.
My start point for the day is Billings, Montana, which has a pretty limited number of flights, and I think that's what's causing me some constraints.
On the plus side, it looks like Delta could interline my baggage from Billings right through to LHR, as Delta seems to have backed down from their plan to stop interlining between separate bookings. BIL-MSP-PHL would have been a single flight ticket.
Anyway, based on all these comments, I'm planning an alternative. I'm going to get out of Billings the previous evening and overnight in Denver. I can then get US481 to PHL, arriving 4:40pm, to leave me over 5hrs before the 10pm BA flight out. If anything, this seems like too much time.
I'm guessing a transfer from US domestic to BA transatlantic might be easier than my original plan, since they're both in Terminal A at PHL.
Some more questions then:
Are US likely to check my bags from Denver right through to London Heathrow, even though it's two separate bookings, but considering both are OneWorld members?
If not, do I still have to go landside at PHL to baggage claim and then bag drop at BA, or are things different now since I'll be arriving and departing Terminal A?
If bags will be checked straight through, should I be aiming for the earlier 6:10pm BA departure, giving me 90 mins to transfer within the same terminal? I'm inclined just to kick back and use the lounge, but I wonder whether on that kind of timeline it would be normal just to push through on the earlier flight.
Thanks again for all the excellent advice so far.