It depends when you want to eat it. The usual way is sow in May to eat in October. (So - end of June, you might just get away with it if the weather's right in October/November.)
But, a tip I got from Wisley years ago: they sowed in October to eat in May! May's quite a useful time for getting green veg. So I've done it ever since....
To put a bit of detail on that: sow mid-October; prick out the seedlings into 3" pots & over-winter somewhere under unheated glass. Next year, as the plants start to grow, pot on into larger pots; then - as early as you dare - plant out under cloches. I use the plastic cloche-hat type, and as soon as they hit the top remove the cloche. (Use the cloches to cover the next batch and you have a succession.)
But bear in mind where I am gardening; you might not get away with it further north. Worth experimenting, though.