I suspect that it's at least partly because the Polish diaspora in the US has longer history
You are. right, but most of those Pole Ams are grandchldren of the immigrsnts.Like my kids.
They feel an affinity to their Polish history, but are not really Polish.
My parents were post WW2 immigrants to the UK.
The Polish government in exile was based in Kensington and Hammersmith and most Poles settled in a similar area, then moved out to Acton, Ealing, Harlesden,Willesden, Cricklewood and later the Harrow area.
There was a small Polish community in South London too, but I can't remember where.Balham maybe?
The Polish community in NW.London in the sixties was not huge, but big enough.
We had a small Polish deli in Harlesden that, even back then would deliver to our family house in Harrow Weald.
His name escapes me.
But there was also a huge importer,with a warehouse in Harlesden called Fregatta which could supply all your needs, cheaper and even in communist times.