In the past, I've always started off a lot of stuff early, peppers, chillis, aubergines, tomatoes, things like that. Last year, as an experiment, I also sowed some more tomato seeds in a pot in my open coldframe in late April/May time. Guess what? They're right
They caught up with the early sowings and overtook them. Nice sturdy plants that never needed to be buried deep to combat legginess. And if I'd started off all my tomato plants that way (all 6 varieties, about 50 plants) it would have saved me an awful lot of windowsill space. I still think you need to be early with things like peppers and chillis, because they were never intended for our climate, but that's the last time I start off tomatoes early.
This year, for the first time, I want to try some onions from seed instead of set. It seems a lot of people start them off in Jan/Feb time. Is that necessary or is it better to wait a little longer with those too?