Maybe it is different in the UK, but in the USA, writing a representative is pointless. A letter from a voter or even a huge pile of letters is not going to change that representative's vote. We basically elect automations who are 100% going to vote a certain way on each issue. A representative is basically an immutable associative array of issue->vote that we add to the legislative algorithm on election day. The time to affect legislation you don't want is on election day. Once your district's particular array is in the algorithm, it is pretty much const until the next election.