This pdf has a useful pic.
http://www.wigan.gov.uk/Docs/PDF/Resident/Planning-and-Building-Control/BCGuidanceleaflet4a.pdf
The relevant quote being "No restriction, but not attached to the house".
This seems to be about Building Regs. What about Planning Regulations?
You are right. the distance in mm is not written down in the legislation. Therefore we have to rely on case history to set precedents. In the lack of there being any other better appeal cases being available 2.5cm will have to do.
Which way did the appeal go? Was 25mm deemed to still be attached? Or was it enough for it to be 'detached'?