I think you are overplaying your hand Merlin.
As iniquitous as the current system is, the politicians have shown that they have no motivation to put themselves out for the 500,000. As Nick says, not a vote winner and there is little empathy from UK residents, most of whom would say that we have made our beds so must lay in them.
A good proportion of expats have been disowned by families back home - or at least have minimal contact. I would not dream of trying to enlist the support of my family on this matter, they have enough of their own issues to deal with.
Remember also that there is a growing opinion that pensioners who choose to live (and spend their money) overseas should not be entitled to receive the state pension. The precedent for this was set with Bereavement Payments, where a claimant had to be domiciled in the UK to claim.
I am afraid that this is a lost cause, the courts have consistently judged against the claim for parity
Nevertheless, I shall continue to add my name to any petition that is doing the rounds