If you do tell HMRC then if you make a loss (expenditure v income) simply because you bought more than you've sold you can set that loss against any other tax. Maybe that is why the govt made that rule, they were missing out?
Easy enough to get a business account from a bank then put all your income and expenditure through that and keep separate. easy then to do accounts at end of year. If you do online you have to do all the sums but the actual page if you have made a small profit or loss is only one or two pages as I recall from my days of doing that, and they don't ask for all the figures, just totals (but keep the paperwork just in case)