You need a CPH number (for the land) from DEFRA and flock/herd numbers for any livestock you plan on keeping (from your local animal health office). As Oaklandspigs says, single farm payment is more complex, but the Rural Payments Agency will help you with that and can tell you about any more grants you may be able to apply for.
If you're going to sell food from your smallholding you need to register with your local food hygiene people too.
Bloody red tape, Grrr!
Best of luck, anyway!