Tails in cavity wall

[Ian1981]

OSG 2.2.3.1 Consumer's Tails, 2nd paragraph. '...and the minimum cable size should be 25mm"

That is not a regulation, mearly guidance going to a default csa for fuses that are rated at 100 amps.
There’s no such regulation preventing you using say 16 mm tails as the fuse could be rated lower than 100 amps

 

[Bobster]

OSG 2.2.3.1 Consumer's Tails, 2nd paragraph. '...and the minimum cable size should be 25mm"

The OSG is a guide, nothing more. It is not regulation.