Could you overnight in HEL on a Saturday so you can take advantage of an 'I' fare? You're still back in London by 9.30 a.m. If you did a back-to-back to DUB/AMS etc. in the morning, you could be back in London by 11 a.m. Saturday, fly out to HEL at 6 p.m. and be back in London again by 9.30 a.m. Sunday morning.