I wish you well........... I don't however expect BALPA to waste their time and my money on a fruitless task, the CAB would tell you your wasting your time, but I don't expect you to belive me.
Pre pay deal the company wanted 737 rated pilots(earning more cash and with better T&C) based at LHR to work albeit on a temporary basis at LGW, the only thing that stopped the company was BALPA intervention, after various discussions it came to light that what your alluding to is perfectly just.
If you do not like my CFE V EOG example, then I draw your attention to the various cabin crew pay scales at LHR where a junior on the new contract will earn £6800 less than a junior on the old contract same job ,same base ,same $hit different pay.