Sorry to here about your bad experience. Unfortunately the best you can do here is learn from the experience (and the advice here) and make the best of the opportunities in the future. A few points:

i) Whatever they tell you, all WT+ fares can be MFU'd (except occasional sale fares that specify no MFUs)

ii) MFUs book into award availability, not any available seat. You should always check in advance and book flights that have such award space. On some routes it opens up a week or two prior but, to keep it simple, it's slim-to-none for SEA-LHR. And on your route 6 months may not be enough.

iii) If you aren't happy with the service (were you led to believe you could upgrade straightaway?) come and query it on Flyertalk. We'd have told you your chances are slim, and that your ticket is refundable if you don't want the risk. And you probably could have rebooked in T (cheap WT+) taking less of a hit/chance.