These questions have originated from the community gardens I support and Twitter. Its a good question to ask just as we are on the brink of the glut season. Times will be good for the next 6 weeks or so, but if we don't plan now there won't be much to eat from the garden come early autumn.
I've also heard it mentioned that we Brits are very good at sowing seeds in spring but don't tend to keep the habit up through out the year!
So here are my suggestions:
Red Russian Kale
Turnips (for autumn crop)
All types of salad can still be sown, especially with the wet weather we've been experiencing
Corn Salad (for winter crop)
Pea shoots (for summer salad crop) - in a week or two you will start to get shoots big enough to add to your salad
Spuds - quite like the idea of eating some home grown spuds with my Christmas dinner. Home grown roasties...yumm!
Carrots (there are many more colours than the orange variety we find in the shops - I'm trying Pennard Plants rainbow carrots this year)
Real Seeds have also prepared a selection for summer sowing. Here's their Summer Sowing Ideas