Covenants are perfectly legal and cover all sorts of things from other people having rights to cross your land or have septic tanks or even TV aerials on your land. As daveO says you may also own, or not own, riparian rights, mineral rights etc. ...
Interesting. I had always thought that all "underground" minerals etc were owned by the Crown - ie Freehold didn't entitle you to any coal for example!
Edit - I was part right -
[h=1]Legislation & policy: mineral ownership[/h] [h=2]Mineral ownership in the UK[/h] In the UK ‘minerals’ are defined in Town and Country Planning legislation as:
‘all substances in or under land of a kind ordinarily worked for removal by underground or surface working, except that it does not include peat cut for purposes other than for sale.’
With the exception of oil, gas, coal, gold and silver, the state does not own mineral rights in the UK. Generally minerals are held in private ownership, and information on mineral rights, where available, is held by the Land Registry together with details of land surface ownership.