Claire, talk to some others in the UK about what you can deduct on taxes and other expenses. Here in the US, I keep track of my mileage every time I run an errand for my shop (post office, buying supplies, etc.), any license fees, all internet and telephone fees as I must have then for my business, office supplies, etc. Also, I use about 17% of our house so I can deduct 17% of the taxes, insurance, utilities, maintenance, etc. When you add all of those deductions up, you can often show a loss unless your business is doing very well. But, as Nana said, you can only show a loss for a few years, then it is considered a hobby. Just make sure you keep receipts and records for everything.
One other down side to showing a continuous loss in a home based business is that you are not paying into Social Security and when you get ready for it, they base how much you get on how much you actually paid in during your total income years.