Flowergirl, I was in our local garden centre today and they were selling off bags of onion sets, potatoes etc. Given how cold and wet it's been you'd still be fine planting these - OK, so your onions may be slightly smaller or ready in September rather than August, but so what? I saw some bags of mixed onion sets - really good idea!
Still plenty of time for purple sprouting broccoli (but be quick!), cabbage, sweetcorn, peas, mangetout, spinach, chard, leeks, carrots, beetroot, turnips and swede as well as summer salad crops.Plus lots of others, no doubt.
To be honest I'd just plant anything you think you can use - seeds are so cheap you will almost always get your money's worth! Do not be too bothered about sticking to the rules - there aren't any really!!
Just reread your post. For winter veg go for leeks, psb, swede and spinach if you have room. Personally I would never omit psb - even our kids will eat it.