MartArt, you can do as you say, HOWEVER, the cable MUST:
1. Run along a permenant wall (not a decorative wall. permenant means it forms part of a larger structure, a garage, for example)
2. Be COMPLETELY enclosed by the conduit, and all joints cemented and the conduit properly supported
3. Use black CONDUIT (round stuff), not TRUNKING. Black conduit is designed for outdoor use and is more tolerant to UV. Use the correct sized conduit boxes.
It MUST NOT run underground, or along the ground, or anlong a fence or decorative wall.
After all this, it might be cheaper or more suitable to use SWA cable, which can run underground.
2.5mm² cable must be protected by a fuse or circuit breaker no greater than 20A. If it is spurred from a ringmain, it must be done using a fused spur with a 13A fuse.