Another one who agrees on the tomatoes. I sowed mine some time ago - they're decent little plants now. If you look at this page:
https://www.gardenersworld.com/offers/ and scroll down, there's an offer on tomato plants - 6 free plants, pay £3.35 postage. Remember if you order anything else, their normal postage charges kick in. If by any chance it tries to charge you normal postage as well, just re-enter the code that's shown on the offer. You can order more than 1 lot & pay £3.35 per 6. With potatoes, you can plant maincrop ones now. mine are planted already but I grow them in sacks, and they can be planted earlier. You'll read about 'chitting' potatoes (forcing them to sprout). I tend to do this with my early potatoes, but I don't bother with my maincrop. Some gardeners reckon that if you put a chitted potato into cold soil, it hinders the growth. One thing - you've not said where in the country you are. This can make a big difference to planting times. I'm in the north west, and always plant about 2 weeks later than friends in the south. Have you thought of growing some salad veg btw? spring onions, radish, lettuce can all be sown now.