I also think a new/better solution must be found. Your contract is wrong and takes you to a dead end road.
I also think you'd have been better off with the mainline contract.
"Some facts to put the situation in context:
The UK pilots are employed by KLCuk. They are subcontracted to work for KLM Cityhopper bv. KLM Cityhopper bv does not employ pilots itself; they also subcontract pilots in from KLM RDA."
You should also add that KLC does not have their own flights, but is operating KLM flights, which KLM pays for.
KLM has indicated to the VNV they wanted to get rid of the UK bases and the UK contract. To facilitate KLM and those who rejected the mainline contract, the VNV gave permission to UK contracters to do not only flights from the hubs but also flights from AMS. I always thought this was a temporary solution to allow people to look for much better jobs with Ryanair, etc..
The UK seniority list applied to UK aircraft and UK contracters. Wake up they are both gone cq disappearing.
The VNV cannot simply give permission to put you high on the KLM seniority list, It will have to ask its members if they mind that somebody is put ahead of them. In the time of the Flight Engineers and the Heli pilots a great deal was given. But if you ask those same members now, you'll have to come with a very good story. You'll now even have to ask your former UK colleagues who did join mainline if they mind to be bypassed. (why would they do that?) they have already heaps of people behind them.
However I do realize all the shattered dreams (what happened to flying B767's out of UK hubs to the US, etc..). I also think your situation totally sucks, without you being guilty for that (although it was a bit dim not to take the mainline contract). So we are all together in it (you a bit more then we) and we all will have to look for a way out.
See you around..