Your best bet is to have a chat with an accountant. I did and he gave some good advice. If you do start trading as a business, then you are supposed to register your company within your first three months. However, if you already have a full-time job it might not be worth your while at the moment especially if you're making little. I was in the same situation, and was advised at the time not to trade as a business. The reason being that I would be earning nowhere near as much as I would be claiming against my equipment, travel, expenses etc. Therefore, as I was paying tax through my full-time job, I would be getting a decent pay-back from the Inland Revenue from the tax I'd already paid. And they don't like that! Accountant's argument being the tax man would be better off, at that time, by me not claiming. If I did it could be seen that I was deliberately not making much money through my photography business so as I could claim against all my expenses and end up getting a tax refund.
Hope that makes some sort of sense!
Talk to an accountant though. Yours may take a different approach.
David