As you're in Reading, you should be able to sow quite a few things in March (provided the weather is not too beastly). I normally sow lettuce, spring onions, rocket and radishes by the middle of March, early carrots and Boltardy beetroot by the end of March. I started the lettuce and spring onions in modules on the window sill, all the rest was sowed direcly.
Last year I planted all my spuds by the end of March (I'm in London). Never grown shallots before, but I think they can go in even earlier.