I have found the following plant nurseries to be very well-stocked with interesting, good quality plants:
Ashridge Nurseries
The Beth Chatto Gardens
Botanica
Crown Nursery
Madrona Nursery
Majestic Trees
Middleton Farm Nurseries
Phoebe’s Garden Centre
Real English Fruit
Stone Green Nurseries
The Place for Plants
The Potted Garden
Woottens of Wenhaston