I connected in the other direction last year.
PDX-SEA AS2126 (arr 1825) > SEA-LHR BA52 (dep 2125).
I had to collect bags, change terminals and recheck with BA. I asked AS about interlining my bags, but they refused as my BA flight wasn’t ticketed through AS. They said that they’ll only transfer bags if both the BA and AS flights were on the same ticket.
When I got to the BA gates an hour before scheduled departure, the earlier flight (BA48) was still there, it had been delayed arriving. BA48 should have been leaving when I was collecting my bag from the AS carrousel.