This year I will have no tax to pay. My earnings ( a meagre £11K) minus some expenses take me below the personal allowance. usually I spend ages working out all allowable expenses as they would include a % of my home as my work is split between office and home office.
Is it worth working out all these expenses when the 'easy ones' - rent for premises, insurances, fuel, professional fees, phone etc - take me below the taxable amount anyway?
Do you claim tax credits? Because if so then I would claim the max expenses otherwise your income will be overstated for that. Also (as far as I remember) exemption from national insurance depends on being below a much lower limit that tax.
HMRC are known to compare one tax year against others, so if your expenses are suddenly a lot lower and out of proportion to income, it will mess up their "year on year" comparisons. It may not trigger an enquiry this year, due to lower expenses, but think about next year. If your tax return suddenly shows much higher expenses claims, it could well trigger an enquiry with a tax inspector wondering why your claims are higher and out of proportion to income.
For the sake of an hour or two of time to work it out more accurately, I'd certainly do it properly. An hour or two now could well save a lot more time and hassle in future should your return ever be picked for an enquiry (more likely if the figures don't follow regular patterns).