Actually you may (according to the electricians' On-Site Guide) use ordinary
T&E "on exterior surface walls, boundary walls and the like" without further
protection although "protection from direct sunlight may be necessary. Black
sheath colour is better for cables in sunlight". Also "additional protection
may be necessary where exposed to mechanical stress" (common sense really).

I have some ordinary T&E - grey or white sheath, I forget - feeding a PIR
light on a south-east facing outside wall for the past 15 years or so and
it's still in perfect nick; there is about a metre from the hole in the wall
to the light fitting.