Not all votes are worth the same in the UK's current system of representative democracy. According to this ITN report some marginal seats have votes with 500 times the power of those in the safest seats. You can enter your postcode at this site, to find out how valuable your vote is. It may make a difference to your decision to vote tactically.
Under the proposed Interactive Democracy system, even if winning MPs are "first past the post", policies must be approved by the electorate in a referendum where each vote has the same power as every other. This is much more democratic.