Immigration may be required, if arriving to an “E” gate and departing from a “C” gate. Twice, actually - one to get out of E gates and another to get into C gates. But it’s a straight forward and swift thing, especially if only transiting. There is definitively no customs to negotiate, but I don’t know about transit security for US arrivals. Both the “E” and “C” gates have the necessary facilities available.

I’ve done SK925/926 a few times, and it’s even chance it’ll arrive at an “E” gate. On the other hand, it’s always been on time or a bit early.

55 minutes would probably be a bit too tight for my taste, but there are plenty of departures with BA and SK to Heathrow during the day if you miss your connection.