Surfing in the south west of England is a year round affair. Winter brings massive almost continuous swells from the North Atlantic, whilst spring and summer has smaller waves but much much better weather.
The best times though can be split into two. September and October benefit from the water being still warm, and some early winter swells. If you’re a serious surfer then it’s the time to go for the best chance of decent offshore waves.
For more mellow but still good waves, then spring is a great time to hit the south west. May and June often have great weather and surf, but without the intense crowds of the summer. July and August are not the best times, the crowds descend and it all gets a bit hectic. However if you want to party as well, then it is most definitely the time to be there.