Word on the street is up to 150 s/o's based over there in the future.
While that may end up being true at some point, I'd not expect that to be the case for several years.
The base is initially set to be 20 Capt and 20 F/O's so presumably ~40 S/O's.
If PER-LHR proves to be a hit then that may well increase but I believe they have other plans in store for the next couple of jets. So there will still be plenty of 787 S/O's hired to the east coast over the next 2 years.