Using JER and INV as starting points (as DVT, teledude and nufnuf77 suggest) brings the July/August SEA fare down to prices that are similar to the Dec/Jan fare ex-London.

So if anyone is looking for a July/August trip, you can do JER-SEA return for 1115 GBP. There are probably better prices and combinations out there but this is from July 29-Aug 1 (US domestics classed as business, and via ORD so far from ideal in TP terms, but sure it's possible to do a lot better):

https://www.google.co.uk/flights/#se...=ONEWORLD;eo=e

(link and date edited as previous version had JER-LGW in economy)

I can also see JER-SEA-JER in First for 1515 GBP in July, which is not bad
https://www.google.co.uk/flights/#se...=ONEWORLD;eo=e