I would like to substitute teach in the English school system this summer.?

I currently teach in American schools, have been for 15 years and am certified in several areas (general PK-8, reading, TAG, technology). I will be visiting a friend near Cambridge this summer, staying at her house for June and July. I thought it would be interesting and possibly a good way to earn a little extra money while I am there. Is it possible for American teachers to substitute teach in the UK? How does one go about doing this?