Embarrassed here as I should know such more, as I do this route a few tiems a year instead of taking the direct BA Shuttle from LHR to SEA.
- Make sure they tag your bag at LHR check-in all the way through.
When you land, you will have to clear customs/immigrations. You can then follow the transfer route, which will allow you to drop your bag back off to make its merry way to Seattle.
Then you will need to check-in again, and make your way to the other terminal for the Uited flight.
Not really much in the way of fancy restaurants, and no lounges to use either with a Priority Pass
. There is an okay restaurant on the back wall to the left of where you come in - reasonable prices.
Yes the immigration line can be long, but flying in W should help as you should beat the stampede that ensues. Luggage should arrive pretty quickly, as LHR normally do a good job of loading the J/W baggage into the right separate container.
Hope this helps.
Thanks,
Richard