Whether it is useless or not depends entirely on your constituency - if you happen to reside in one of the close-fought marginals, your vote in the current circumstances is actually quite important.
If you're in one of the terribly one-sided constituencies, it is pretty much worthless.
That said - if half of the non-voters who thought voting was 'hopeless' came out and voted in the 'one-sided' constituencies, I suspect there would be a lot more marginal constituencies out there.
So it's worth voting - even if your chosen candidate doesn't win, because in a future election it might make your constituency seem less hopeless, and therefore prompt more people to vote, and increase the chances of your chosen candidate/party winning in the future.