There are probably some plots where the owners have overwintered veg (they have been in the ground since Oct/Nov last year). For example, onions, garlic, broad beans. Also, some purple sprouting broccoli and other brassicas might be left. I also have some perpetual spinach still on my plot and has been there since last June.
You can get catalogues from any seed company, eg Marshalls who are quite good. Order a lot of catologues, don't order anything from them, and then they usually send you a £5 off your first order voucher!