www.porthkerris.com
We would highly recommend Porthkerris Divers in Cornwall.
They are by far the most professional operators in that part of England. You will need to take your log and a qualification card with you, or they will not let you dive. You can stay in local country pubs at low costs there too.
their address is
Porthkerris, St. Keverne, HELSTON, Cornwall, England, UK. TR12 6QJ
Telephone Outside UK: +44 1326 280620
At that time of year you will see basking Sharks, Blue sharks etc.
their site is
http://www.porthkerris.com
If you are looking to do inland diving, we do guided dives around europe's most infamous inland dive site at Stoney Cove in Leicestershire. It is only 40 minutes drive from London.
If you would like more information, feel free to email me or PM me.
But given a choice, I would go to the South Coast
Best of luck
Ian