Running 10mm T&E externally?

[Toneyz]

And there is nothing in BS7671 that I'm aware that would prevent using HO7 flex for the complete circuit.

The only reg that comes to mind is adequately supported.

 

[DPG]

Galv seems a bit excessive for domestic, but depends on budget I suppose.

 

[Mark42]

Have you considered using NYY? I love the stuff: a bit dear but dead easy to install (unlike SWA) and it knocks spots off T&E.
You could do the whole run in NYY - no need to change cable types, and being solid core not stranded like HO7RN, it's easy to terminate at both ends.
I've just used some 5-core 16mm NYY for a sub-mains in my own house, where I'd have used SWA before. I'm converted! (Obviously in an area not liable to mechanical damage.)