Originally Posted by
flarmip
Provided the OP has a US passport (or one from the several other countries who nationals can also use the e-gates), even the "standard" immigration process is extremely painless compared to what you get in the US.
The final destination is Italy though, so why bother going through the UK at all? They would have to clear UK immigration, then EU immigration. Why not take a flight from SEA to somewhere in Europe (CDG, AMS etc) clear EU immigration when they land and then it's home free. Given the congestion in LHR and London generally I can't see any advantage to touching the UK at all, it simply becomes a transfer point that has no advantages compared to a transfer point in mainland Europe.