Not in UK, but rule of thumb seems to be about two months before last frost is the minimum. I think it is more a matter of what your goals are and what sort of indoor set up you have. Available space and light make a huge difference. I will be starting late this month and things wont go outside till March. I grow, top, clone, move all winter long. The older the plants are when they can live outside, the more they produce in a single season.