Good news-United does do a free stopover but bad news-you have to book the trip all together so you'd have to wait a bit longer for the stop over and flight back.
Say you're wanting to fly Friday, June 5th from DTW to BCN (layover in EWR), that's 70k x 3; then BCN to LHR on June 13th-that's 0 miles, home to DTW from LHR on June 16th that would be 66x3= 408,000 plus $683.79 in fees.
It would definitely be cheaper if you positioned in EWR -270,000 miles and $666.99 in fees (LHR kills you on fees). If you wanted to avoid fees I'd recommend asking your friends to meet you somewhere in Europe lol.
If you flew out of ORD and didn't mind flying TAP Portugal, that would be the cheapest (miles wise) at 180,000 miles and $709 for ORD-BCN, BCN-LHR, LHR-ORD.
Anyway, hope this gives you some ideas!
It's kind of a wash (to me) whether it's worth it to do the free stop over and risk having the flight I really want when they open. Everyone is different-I know there will be lots of flights on certain routes so I'd rather get to Europe and back the way I want and then fiddle with intra Europe later but everyone is different.
Are you cruising with
DCL or another line?
Do you know where you're staying in BCN?