Hi, as it's currently in a pot, you can plant it at any time the ground isn't frozen or waterlogged, make sure you keep it well watered, they like a LOT of water, Derek.

If you plant into the ground now make sure you water really, really well; especially if you live in the southern part of the UK. To be honest I'd wait until early September to plant in the ground, less stress for the shrub and easier to keep watered.