Woottens of Wenhaston, Wenhaston, Suffolk https://www.woottensplants.com/
A real plantsmans nursery with a wide selection of plants, specialising in iris and auriculas. They have limited opening hours so check the website and can be a little on the pricey side. 

The Plantsmans Preference, South Lopham, Norfolk. https://www.plantpref.co.uk/
Small but well stocked and managed nursery. The owner is very helpful and knowledgable. They have a wide variety of grasses and all their plants are grown and raised on site. 

Botanica Plant Nursery, Campsey Ash, Suffolk. https://www.botanicaplantnursery.co.uk/
A large nursery and arboretum, you can get lost here! A wide variety of plants and amazing selection of trees of all sizes and ages. Well worth a browse.