Worth a go getting seeds via the seed distribution from the Royal Horticultural Society or the Hardy Plant Society. You have to be a member and there is a small charge but you get some interesting plants.
Buy/Pre-order in advance regardless of sowing times and then store until needed. As JennyJ says, it’s all about timing. Pre- Brexit I used to browse at the Organic Garden Catalogue soon after Christmas and place one big seed order for the whole year. I always got the varieties I wanted. The only exception was seed potatoes for which I had to pop over the border to France to buy from a GC.
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
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https://www.realseeds.co.uk/