Unless you are into competitions (and even village hall comps are intense!) then I would stick to things that you will actually use. Leeks are always reliable and great for soup, toms are a longtime favourite but need a lot of care and you'll probably end up giving half of them away, you can't beat new spuds straight out the ground and cooked within the hour and if you've ever tasted strawberries on a warm summer's day right from the plant then you'll never want to buy another strawberry again. Mmmm ... delicious. If you like 'em, raddishes are also quick to grow and all you need is a shallow drill, thin 'em out then plant another drill for a sucession throughout the salad season. Repeat for as long as you desire.
Fresh herbs are also much prized by anyone who does the cooking so tyme, rosemary, parsley, dill and chive will aways be uefull. Bay can also be kept outside during summer but provide protection and shelter from cold winds in winter.
For the rest it's hardly worth the effort and I would delay planting 'till the garden soil temperature reaches 7C, even if you are sowing in a cool house or frame. By the time your seedlings are ready to go out the ground should then be nicely warm and they will soon catch up with earlier sowings which are likely to either wilt, bolt or damp off. Seed packets tend to be orientated toward the SE of the country and for the likes of Scotland you can add 4-6weeks to the recommended sowing times, Cornwall you can probably get away with planting earlier than it says on the packet so it does depend on your area. When harvesting, sprouts and root veg like parsnips or neeps they always taste better after they've a hard frost or two in the autumn.