The decline of party membership in the UK
Figures are available for 2011 party membership of Labour and the Liberal Democrats (but not the Conservatives, who seem rather secretive) here:
The Conservative party is estimated to have a current membership between 130k and 170k, so for the sake of this graph, I put the middle value of 150k.
I also used data available here:
With all major parties in a long term decline, is party system democracy becoming a thing of the past?