Extremely illuminating site here, which shows the relative value of an individual vote. Input your postcode (case sensitive) or constituency, and statistical analysis provided by the New Economics Foundation does the rest.
I'm in a safe Tory constituency, and the relative value of my vote is 0.0082. As if we didn't know, only voters resident in marginal seats (about 20 percent of all constituencies) have any impact on elections. I don't, so I won't bother.