Malicious compliance! I admit I didn't realise that could even be a thing with regards to tax (for the most part that would suggest to me the rules aren't fit for purpose, as likely in this case)
Whole thing is currently putting me off from trading as I can't see how I would work out the tax other than you describe (which is indeed a nightmare).
Happy to pay taxes (ok, happy not really the word), but would be more sensible to send a list of gains/losses for assessment and then be told what was owed. As opposed to trying to figure it out with little to no guidance from them only to be then later be told its wrong (or worse being done for avoidance!). Guess I'll try and talk to an accountant, although initial contact indicates they don't really know. (A call to HMRC wasn't exactly helpful either).