I use Seed Co-operative and rate them very highly. I happen to live only a few miles away from where they moved to not that long ago. They have interesting open days where you can wander round and see their growing experiments under glass and polythene, their seed sorting equipment and so on. They grow much of their seed themselves and source others from a small handful of organic growers. They also uphold biodynamic principles. So definitely very pure in their philosophy and practice! The varieties they maintain tend to be fairly traditional, open pollinated but perhaps not highly adventurous.
I've also used Simply Seed and find them fine too.