This is not practically the case. Take Bath for example (I happen to vote there, since it's the last place I lived - prior to that I lived in North East Somers

This is not practically the case. Take Bath for example (I happen to vote there, since it's the last place I lived - prior to that I lived in North East Somerset where the dynamic is the same, but with different parties). There has not been a member elected for Bath since the 1800s who was not either a member of the Liberal Democrats, the Conservatives, or a party which became one of those. The margins are usually comical. Therefore, one must vote according to who they want to prevent winning if one is not for either of those two candidates. Realistically, the only hope even for _Conservatives_ in Bath in the modern era is that a general election is held during University holidays.