I like Realseeds, but they have a very limited range, plus a minimum order, a nuisance if I only want to top up by a packet, when seed-saving fails for some reason.
Chiltern have a fantastic range of garden plants/trees/flowers - but not a huge range of veg seeds IMO. So depends what you want to buy.
I regularly use Kings, they weren't as quick as usual with delivery this year, but I think they had Covid problems along with everybody else, so will order from them again this year.
I also like Francchi/Seeds of Italy, when I used to look after the school garden, once registered, they would send free seeds to the school each year, to encourage children to grow and eat veg. Great company ethics
There are several companies out there that I just refuse to use, because service has been terrible for more than a few years. Enough is enough