As a Trained Hunter you may sell to a local butcher, restaurant, etc. but only as Primary Producer, i.e in the fur, with no further processing beyond what a hunter does in the field, or maybe in a larder.
Or pass it on to friends and family after you have cut it up yourself, no further than that. No money may change hands.
You don't even have to be a Trained Hunter to do this. No qualifications are required.
Some local restaurants, pubs, and butchers etc., are set up to accept such primary produce, from whoever, then process it themselves. but that is an exception. However you are always responsible for supplying safe food, whether you are qualified as e.g. a Trained Hunter, or not at all.
Anything beyond that, such as supplying an AGHE, or just skinning it, nevermind cutting it up, or selling it to the public on a market stall, or from your door means registering as a food business (or working for an estate who are set up appropriately), and having certain measures in place, agreed with your local authority.
All explained in the Wild Game Guide, make sure you are reading the current version.
BTW, £7/kg sounds like the sort of price for meat, not primary produce, AKA a complete carcase, unskinned. Maybe some retailers would take such stuff, no questions asked, but that would be breaking the law, which has not changed for many years.