Fruit & veg
I can see you're targeting the Amazon Prime generation who buy everything online. Hello Fresh-type services where people need help just compiling ingredients for a meal! lol.
I know these people, they're my sons and daughters-in-law and they don't have a garden, just a balcony. They also don't have time so your service is a time-saver for growing a few pots on their balcony / windowsill?
It is a good idea, but with some of the tweaks already mentioned. You would also need to budget a fair amount for marketing, going by the amount of catalogues and emails I get from horticultural companies.
When I got the itch for gardening I lacked the confidence to do seeds at first. I used to buy plants, but as they got ever more expensive I switched to seeds which was much cheaper. That might be a good thing to exploit in advertising your type of business.
The "recipe box" principle
could work quite well - give all the things you need to pretty much guarantee a healthy plant and a 'recipe card' to follow to on how to grow. Save the cards or pass them on. I see the same subscription box principle being used to send monthly beauty products. And I seem to remember something similar on Dragons' Den a while ago.
I can see potential, esp for younger people with no experience and little outside space.
Perhaps aimed at children too.
Various companies do pre-packs
already - seeds/bulbs, soil, pot, label, pencil etc. The ones I see are more aimed as gifts/kids rather than anything that would genuinely thrive using the bits provided; the pots are often tiny with no drainage holes.
Houseplants are in fashion, I understand. Perhaps growing those from seed/bulbs could be fun and not so seasonally tied (?).
Succulents are are more drought tolerant often - perhaps practical for business execs would travel a lot.
I agree that spring is the mad seed time. Seeds would certainly be the lowest cost option, over bulbs, plugs etc. I agree it would have to be aimed at folks with no gardening experience at all. The rest will hoik down to the garden centre.
If you could sell kits/boxes for £5 you might be on to a winner, if you can figure out how to find things that would be easy, successful, in demand, good for the time of year. Also that you could provide without enormous infrastructure, storage or overheads.
My concern is whether it's a viable business model.
Too many variables would imply a lot of processing, ergo, lower profits.
Keep us informed if you go ahead.
only if they were really particularly rare seeds/plug plants would i even think of paying that much