Hi,
The best advice is to plant what you know you will eat. There is no point planting a row of Beetroot, if no one in your family likes them.
That said, at the moment I have some 1st early potatoes ready to go in, and broad beans can be planted now, runner beans are always a favourite in our house (plant end of April/May time) and Sweet corn was a great success last year.
I have often found the most cost effective veg to grow is Brussel Sprouts. But I can only do these every other year as I only have a small back garden and we have a seemingly infinate number of rabbits on the allotment who eat them before I can.
Chatting with fellow allotmenteers is a good option as they will know what grows well on your site and may also give you some of their surplus.
Good luck with whatever you decide to grow.
D xx