Fantastic stuff PP, it really is the most enjoyable thing in the world.
You've missed the boat on growing Tomatoes, Chillis and Peppers from seeds but you can buy a couple of plants from supermarkets, garden centres and even from little tables outside people's houses. They do appreciate rather warmer temperatures but i've grown both outside before. One of those small polythene greenhouse thingys are often ideal
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Carrots....lots of different types and colours....you can buy seeds from most places (not the barbers or the furniture shop!!!
) Plenty of time to sow them, sow a small amount every couple of weeks up to August but remember to water them every day unless it rains. Generally speaking, most root crops need poorer, stone free soil so don't go chucking any horse poo at them
Brussels can still be sown and are easy to grow. They will, along with cabbages, need to have a cage over them to stop pigeons and cabbage white caterpillars, I use that blue poly pipe covered with green scaffold netting.
Potatoes....I'd just plant some from the supermarket. They usually take around 12 weeks before harvesting but I was still digging mine up in November.
Peas....chuck them in now and like the carrots, do just a small amount every couple of weeks. They will need something to grow up....a few garden canes with string or wire is fine.
Plenty of time to grow sweetcorn from seed too. They can get vey tall and need to be planted in blocks in order for the wind to blow the polloen around. You'll need around 10 plants.
I usually put my onions in around October time and i've never grown them at this time but saw someone at the allotment had put some in recently. I prefer sets rather than seeds.
You can grow all of that in the ground, in pots or tubs and in garden sacks if you want.
The most important thing is to ask...ask lots of questions however silly they seem, we all had to start somewhere and i'm still learning lots.
Good luck and get growing