Minimum requirements for horsekeeping in UK?

Honestly I don't think it matters that much because I literally can't think of a single place around me where there aren't too many horses per acre. It's a small country, there are a lot of horses. You could definitely get away with doing that- I don't know how it works in the states but although there are laws over here, they are definitely not enforced and if they were, there simply wouldn't be enough land for the amount of horses kept here.