http://www.pickandmixseeds.co.uk/, as the name suggests they have the whole pick and mix angle going on or http://www.singleweedseeds.co.uk/ is equally good. I think that pick and mix may have a bit more choice though.
I've always found that http://www.pickandmixseeds.co.uk/ has fast and discreet shipping. the seeds that i have bought from them are all that i wanted them to be :weed: