The BBC has interesting analysis of the declining membership of UK political parties over the last few decades here.
Political party membership had once been high, but now about 1% of the UK population belongs to a political party. The BBC provides these estimated figures:
1951 Conservative 2.9m - Labour 876,000
1971 Conservative 1.3m - Labour 700,000
1981 Conservative 1.2m - Labour 277,000
1991 Conservative 1m to 0.5m - Labour 261,000 - Lib Dem 91,000
2001 Conservative 311,000 - Labour 272,000 - Lib Dem 73,000
2011 Conservative 177,000 - Labour 190,000 - Lib Dem - 66,000
(Source: Estimates based on party reports and House of Commons Library)
These numbers are nothing like what you find in the US. One major difference is that US political party membership is free. Party membership is not free in the UK: membership can cost as much as £40 per year.
British political parties are becoming the bodies that provide the public with its politicians, but to which the public does not formally belong. This is a worrying development that has become more problematic especially over the last 30 years.
What to do? Well, I'd recommend that readers join me in supporting the Labour Party. Membership information can be found here.