Fresh from our garden at RHS medal at Hampton Court flower show (shines medal... :-) ), we were educating people about a phenomenon in British farming called 'The Hungry Gap'.
That is when our British veg is running out, cabbages, carrots, leeks, turnips etc and we don't get anything back from the fields until the end of May. This is why our supermarkets are full of fresh veg from Italy, Morocco, Spain, Portugal, Kenya etc. We cannot produce enough fresh veg for 7-10 weeks, period.
As a seed company, we tell people that when they pull something up, they need to put something in. The thing to do now is SOW BRASSICA's which are slow growing and will take you through the cold winters (cavolo nero, sprouts, cabbage, sprouting broccoli raab etc), but also to sow Spinach and lambs lettuces in Sep/Oct, Broad beans, peas and sugar snaps in Nov, garlic in Nov/Dec, radicchio and endive are great too as they resist cold. These are cold loving, you don't need to heat them, just keep snow off them, cloche, cold frame, cold greenhouse and there are others.
Sow every single month of the year and you will have constant fresh veg...... here is a list
You can buy a veg growing seasonal survival kit here