Hi Deadduck, It depends what you mean by "register as a smallhold"?
Basically, the rules are that if you keep any farm livestock with four legs, you must obtain what's called a "CPH" number. If you're living in an old farmhouse, you may well find that your land already has one of them, but either way your local DEFRA office will be able to either tell you what your CPH number is, or issue you with a new one. The other thing you need is either a flock number or herd number, to go with your new four legged livestock (that's what goes onto the ear tags of animals you breed for instance to identify them as your animals). Again, you get this number from DEFRA, and it's free of charge.
If you keep poultry, you don't have to register with anybody unless you keep more than fifty birds in total (e.g. 30 hens and 21 ducks), above which you DO need to register them.
There's lots of info on this sort of thing on the main TAS website, so that's probably the best place to start. Registering as a business is a totally separate issue, but it sounds as though that's not what you're trying to do anyway?
Hope that helps!