I would find it most odd if you were instructed to be FL250 by Foyle on a regular basis. In my experience 'slow' climbers are coordinated in the climb every time. There is no standing agreement/agreed level procedure on that route so it is unreasonable to expect anyone to bust a gut to make such a level other than the odd occasion where traffic might dictate it (such as trying to jump traffic routing PTH-GOW at FL230). Even then it should be a request, not an instruction.
Anyway, if you are instructed to make an unreasonable level again, just say you can't do it and the controller concerned will have to adapt his plans accordingly.
Incidentally, a similar situation exists round about the IOM with traffic out of Belfast City. In this instance there is an agreed level between the South West (ScACC) and sector 7 (LACC) secotrs and traffic should be FL290 by KELLY. Alas BMA A321's find this nigh on impossible, especially if there is a tail wind and we invariably have to coordinate the aircraft out of FL260 climbing to 290 with the London boys.