and if you notify your tax office that you will be doing some self employed work, and that your financial year will end on (say) 5th April, then they will not have any reason to accuse you of doing undeclared work. All being well they will remember to send you the forms in time for your financial year end.
Ask for the leaflets on deductables such as tools, overalls, ear defenders, gloves, proportion of running expenses for transport, business stationery, part of the cost of your PC and phone, and stuff. I think it is not worth claiming part of the cost of running your house as business expenses, as that means you could have a capital gains tax liability when you sell it.
If you are careful and methodical and don't mind filling in forms it is fairly easy.
BTW if you choose to put some or all or your earnings into your pension fund (paid straight out of your business account,not paid to you as earnings first), that amount will not be subject to tax or NI, because it is not part of your earnings. (if it is paid to you first, then it is subject to NI but not tax). This is a particularly good bargain if your earnings push you into the higher tax bracket,and might enable you to retire early and live in luxury
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