My advice for growing is that now's a perfect time! As the days are warming, so is the soil but cool April winds and a few surprise frosts means that waiting until May to plant out seedlings isn't a bad idea. My advice is to grow healthy, strong seedlings in pots now, in preparation for planting them out in May. Sowing directly is more risky, meaning slugs and snails are more likely to munch them as they germinate, and small seedlings are at risk of being smothered by weeds.
Good things to grow now if you have a garden / allotment are:
1) Things that keep on giving! Chard, rocket, lettuces, fast growing herbs like coriander and dill, beans, peas and courgettes can all be picked every few days and will keep fruiting allowing you to eat something delicious every day. Sowing rocket, lettuce and radishes every few weeks will give you a steady crop all season.
2) Things that keep you in veg all season: beetroots, spring onions, kohl rabi, and fennel are all slower growing and will happily stay in the ground until you need them..meaning you can avoid a dreaded glut.
Things to avoid if you don't have a lot of space: Pumpkins, more than a few courgettes plants, lots of beans, lots of sweetcorn..these all take up loads of space and don't give as much back for the space they demand!
And don't forget...flowers! Borage, dwarf sunflowers and marigolds are all edible (the petals) and will attract lots of lovely bugs to help keep your veg garden healthy.
A lot of seed merchants are currently either sold out or limiting seed sales.
https://vitalseeds.co.uk/ are a lovely small business in Devon who open their shop daily at 9am.
Tamar organics ( https://tamarorganics.co.uk/) currently open their shop from 9-10am on weekdays.
Please only buy what you need!! Split packs of seeds with friends and family, leaving unplanted seed to go stale is a waste in these times when they're in such high demand.