Why not put in some earlies and some second earlies? A first early we like is Lady Crystal but why not have a chat with some others on the allotment to see which do well for them? I reckon first earlies are the best value as that's when shop ones are dearer - and they tend to have less flavour anyway..Its always a good place to get advice for your particular plot. Unless the allotment is a big one you might find its hardly worth growing maincrop. Sometimes you can get fairly small bags so you can run your own trial and see which you like best.

Plant them when the soil is just beginning to warm up, probably around March.

Consult the others re carrots as they don't like heavy ground. Always fee free to ask as many questions as you like!

Container growing spuds is worth considering imo. I always seem unable to get them all out & get unwanted re-growth the next year

As Stera suggests, best to ask other allotment holders as to the best vegetables and varieties to grow. You will need to consider things like slug damage on potatoes, depth of soil for carrots and parsnips and whether the site has things like club root which will need some attention regarding sprouts and greens. Just bare in mind that if some things don't work out well next year, then there is always another season for success. Good luck!

Ha ha Darren,these are often known as "volunteers" and if you leave them in they usually crop again. (If you are tidy minded and this upsets your crop rotation of course that's different...)