Ofsted have published an analysis of the numbers and proportions of girls and boys studying A-level subjects in England:

Until now there has been no single source of data for schools or inspectors to consult that sets out the numbers and proportions of girls and boys that progress from Year 11 to AS levels and then from AS to A level. This report provides that data, so that schools can compare their own performance against the national picture. Several subjects have significantly unequal numbers of girls and boys, for example physics.

Do read the report for the figures in detail, but Dom McDonald, Programme Manager, Outreach at the Royal Society of Chemistry has the STEM subjects summary:

key "Science" numbers (Girls:Boys)
Chemistry 1:1
Geography 1:1
Physics 1:4
Maths: 1:1.5
Further Maths 1:3
Biology 1:0.66
Psychology 1:0.4

— Dom McDonald (@TheOxfordDom) May 13, 2015

Also on Twitter, he went on to note that the most extreme subject appears to be Computing, with a girls:boys ratio of 0.09:1.

Yikes.