I believe the most suited business structure would be a Sole trader. You submit a note of your profits if any along with your personal tax return yearly to HMRC.
A Sole trader is the simplest way to run a business of this nature. Keeping accounting records are straightforward, and any profits go to you. However, you personally are liable for any debts that your business runs up (which means your assets may be at risk).
There are various expenses that would relate to your sales, which potentially could be offset against the turnover for the year. This would reduce the amount of tax payable on profits at your year-end. For ease, we would suggest your yearend coincides with the tax year-end.
To keep matters simple, you should open a separate business bank account, perhaps in the name of Serendipity2 t/a Serendipity Photos.
You ought to investigate appropriate insurance too!