I don't know the specific detail of OP's position, but (all?) post-docs in the UK are members of the same pension scheme as all other academics in their institu

I don't know the specific detail of OP's position, but (all?) post-docs in the UK are members of the same pension scheme as all other academics in their institution. Your fundamental point is quite right--short-term contracts are a lousy and difficult game to play into middle-age.