I’m very much enthusiastic amateur rather than any sort of expert, but these are my thoughts:
I bought a tray of six small strawberry plants a couple of years ago from b&Q, and since then they’ve produced lovely strawberries and also at least 40 baby strawberry plants. You can grow them in a grow bag, or in pots.
I’ve had success with growing from seed using a propagator. Seeds from Wilko, suttons, or the supermarket have been fine, or a brand called premier seeds on Amazon. The only problem
ive had was growing chives from seed, tried two brands and they wouldn’t grow. Then I collected seeds from a chive plant growing in a patio crack and those seeds germinated.
For tomatoes I grew the ‘sweet million’ type last year, they were very sweet and I got loads. I started them in the propagator in those cell trays with the lid on, took the lid off as they grew bigger, moved them into a bigger pot (9cm pots) until I could see roots at the bottom of that pot, then they went into their final container, which was One of these
For herbs there’s an amazing online shop called Urban Herbs which sell unusual varieties, all the plants I’ve had from them have been really good quality.
For growing seeds I use a seed and cutting compost which is very fine, then once things are ready for their big pot I tend to use a 50:50 mix of normal compost and top soil, and then throw in some extra sand or grit too. Make sure your pots have drainage holes, and ideally use pot feet to raise them off the ground to aid drainage (I use the round black adhesive foam type ones, I got a big bag off Amazon).
Also on Amazon you can get felt grow bags which are for growing potatoes, you roll the sides up as you earth up the potatoes.
When I went in a couple of weeks ago Wilko had blueberry bushes, which grow well in pots.