I was in a similar situation before but opted for the 431pm Eurostar just to be safe. I could have made the 215pm train in the end.

i arrived on BA282 from LAX which was scheduled for 1055am. Got on the jet bridge at 1115am, by the time I started queuing for immigration it was close to 1130. Took about an hour due to the queues and also immigration officers started going for their lunch breaks (it was only about 6 counters open when I was cleared).

Tube took about 1.5hrs, got the king cross at about 130pm. Hope this helps.