AIUI, 2.5mm cable run in contact a conductive surface with the rest
of the cable completely surrounded by insulation is rated at 21A
which is sufficient for a 32A MCB protected ring main.

I don't have my OSG on me either. However, I seem to recall a 50% reduction
for being buried in insulation, which is what you would have to use if using
the OP's original idea of letting it float within the cavity, not clipped.
This would mean his original plan would be seriously non-compliant, quite
apart from the cavity bridging issue.

Christian.