I think Pit Bull has hit the nail on the head with the companys current plans to tour RJ crews through the regions. Given their statement that there are no roles for BA pilots in the new BACE RJ operation, I think they'd do their level best to avoid paying relocation for a move north only to have to pay it again two years later for a southbound move.
Squirrel - BACE do operate 146s with 100+ seats and I don't suppose any scope agreement would require them to surrender those, it's just that they wouldn't be able to buy any more 100+ seat aircraft. 99 seats fine, 101 seats not fine. After all, whats to stop management handing over the 737-500s or A318s next? Under current management all BA jobs are going to be at LGW/LHR because there won't be any regional bases. That won't prevent the company trying to tour you through the regions though. I wouldn't get too hung up with your suspiscion that people think you're going to be jumping the seniority list at LHR in some way. Junior people have been joining long haul fleets for donkeys years and its just accepted because type freezes or insufficient hours prevent more senior people moving. If you wish to go long haul with low seniority then your situation is no different to that of a DEP and I'm certainly not envious of it.