I'm doing SFO-BCN on award travel in biz this July, and routed via London (AA SFO-ORD-LHR-ORD-SFO, BA LGW-BCN-LHR). I have a 3 day stopover in London en route to BCN. The only problem I really ran into was getting home from BCN via LHR the same day. The BA flight BCN-LHR arrives too late to connect to an AA flight to the U.S., so we have to overnight in London and fly out the next day.

I tried to set this up with stopovers in Dublin (didn't work...EI only had 2 flights a day DUB-BCN an no award availability), or EDI (worked if I wanted to depart at 0600, which I didn't)).