Whether you’re trading is a judgement based on ‘badges of trade’. There a number of considerations aimed at deciding whether you are undertaking photography in a commercial way with a view to a profit. If yes then you will be liable to income tax and National Insurance Contributions if you earn over certain thresholds. Even if these thresholds aren’t met, you must inform HMRC within 6 months of the end of the tax year or you may be fined £100 for failure to notify. Registering as self-employed can be done by completing form CWF1 I think or online. Once registered, you will need to complete a self assessment tax return by the 31st January following each tax year.
You ought to have been keeping a track of your allowable expenses to date as these can be carried forward and set against any subsequent profits such as now.
HMRC will look at each case on its merits as they don’t want lots of hobby photographers with no prospect of a profit claiming to set off losses year on year against their other income.