If my disposals amount to say £50,000 (ie more than £40,400) but my capital gains are below £10,100 under what authority are HMRC stating that I must complete the CG pages?
All I can think of is that it is in the CG notes - and I accept that this is not much of an answer. And the notes have always carried words such as "These notes are for guidance only ..."
Looking at it from the other direction, do you not wish to complete CG pages in respect of a particular transaction? If it is land/property or shares, then remember that HMRC receives information from many sources - and is getting better at data matching. So if you don't declare the disposal you may well be at higher risk of an enquiry. Granted, if your net gain is below the AEA there will be nothing to worry about and no tax to pay, but no-one needs the hassle of an enquiry.