Minimum size for consumer tails

There's no specific requirement in BS 7671 for meter tails - it's a matter of applying the general rules for sizing conductors.

A downstream protective device may protect a conductor against overload, but it can't protect it against faults - for that you're reliant on whatever is upstream - in this case likely the DNO's fuse. So in theory it's just a matter of calculating based on the fault current, characteristics of the protective device and the adiabatic behaviour of the conductor.

The other approach is not have fault protection - BS 7671 only allows that in very specific circumstances - conductors less than 3m in length, installed in such a manner to reduce the risks of faults to a minimum and reduce the the risk of fire or danger to persons to a minimum. You might get away with that sort of thing for connections to say a bus-bar chamber in industrial grade enclosure in a fireproof switch room in a controlled environment, but it's perhaps more difficult to meet those conditions in a domestic kind of setting. Note that that get-out is only for protecting the conductors against fault currents - not protecting people against shock as a result of a fault - so likely some care is still needed with regards to metallic containment/enclosures.

The other issue is that the DNO's fuse is really outside of the scope of BS 7671 (and of your design) - and the DNOs tend to like to keep their options open - so an installation that's supplied by a 60A fuse today might be supplied by something else in the future (80A, 100A, 125A even?) or maybe even a circuit breaker rather than a fuse (as is common in the rest of the world) with likely differing characteristics, and the DNO won't want to start having to re-evaluate the on-off designs of consumer's installation every time they upgrade a fuse. Hence many DNOs recommend things like max 3m of 25mm² tails on a 100A fuse or 16mm² on a 60A fuse - and will refuse to (re)connect if smaller tails are found.

   - Andy.

I would use 25mm in line with accepted practice. The extra cost is trivial for such a short length. Whilst a smaller size might well be electrically safe, that counts for nothing if the DNO require 25mm and decline to connect anything smaller, either at the time or later after say a meter or fuse change.