There are some suppliers who either make their own seed or source from UK producers: Real Seeds prepare their own (I’ve found all the seeds of theirs that I’ve used to date very reliable, including parsnip, leeks, onions, courgette, sweetcorn, winter cabbage, tree cabbage, sicilian radish).
Also, price per seed is probably the best metric – I’ve noticed that some suppliers have far more seeds per packet than others – Stormy Hall carrot seeds has hugely more (1500) than most others for one.
The other thing one does start to question, Peter is this: is the way the market is going that of big suppliers growing young plants and selling those to customers? You transplant out your plants rather than sow your seeds?? They may see that as a more profitable solution and many customers may see it as more hassle free, avoiding the challenges of early frosts, the need for fleece etc etc.
It may be that quality seed is going to swing in the direction of success for more niche players supplying the motivated minority, whilst the mass market supplies young plants instead……..