I've had a news alert come through with a little more detail and it says that shops that can stay open are:
Supermarkets, pharmacies, vets, pet shops. hardware stores, retail shops in hospitals, newsagents, petrol stations, banks, post offices, laundrettes and undertakers. You are also allowed to go to work if you can't work from home, but only if you can stay 2m apart from each other. This means that construction sites won't yet have to close, and delivery services can continue. (all just taken from the news).