United don't have a J class, their United Business cabin is either C (full fare/flexible) D (Cheaper flexible) Z (Restrictive/Cheap). Your rewards ticket will probably be issued in NC
class, and I would think that it is unlikely that you would be upgraded when you are flying on a non-rev ticket. If they need to bump people up, it would be those who are holding a full C class ticket, and probably Premier Exec, or Premier 1K mileage plus members.
Having said that, you never know your luck, hope you enjoy the flight. I have done LHR
and back a few times with UA
, and I think they have a pretty good Business product
If you were to get upgraded, it would be best if you were booked on the flights operated by 777 or 744, as the 763's don't have First Class Suites installed.
Good Luck, and Enjoy your trip.