As an EC member, I assume you put this number in to the booking (by Economy+ I guess you mean World Traveller Plus?) when you made it? If so, simply log in to your EC account and on the right hand side next to your account info, hit "Manage My Booking". This should bring up your new booking. From there, on the right again you'll see 'Request a Seat' and bingo, there you go.

Usual caveat: not all seats are opened up for pre-assignment, so if you still can't get what you want (e.g. all seats gone already), make sure you also use on-line check in (OLCI) at 23 hours before your flight which will allow you to pick your final seat (and any non-assigned seats/ones held back which couldn't be pre-assigned before should be available).

Phil

PS Most reliable browser to use is Internet Explorer, users of other browsers have reported problems changing seats.

PPS If you forgot to add your EC number when you booked, use the 'My Booking' feature on the top right of the main BA homepage to retrieve the booking with your surname and booking reference (see the e-ticket receipt mail, it'll be something like XYZXYX) then you can do the same seat selection, and use the 'Add Frequent Flyer Number' option.

PPPS If you're also an AA member and added your AA number, call BA and replace it with your EC number instead! You can't earn/spend miles between the different AA/BA programs on USA-LON flights or vice versa.