How can I convert wall lights to under-shelf lighting?
Yes you can just extend wiring. However, exactly how will depend on the particular circumstances of your wall.

You can get plates that just have hole for the wire to come out of - this one from B&amp;Q or this one, also from B&amp;Q for example - so you could fit one of those. If the wire would get in the way then cutting a channel down to the correct height is the best bet. Make sure you use suitable conduit to protect the wire. You can then fit the plate at the lower level. You can use a blanking plate over the existing outlet, if you have enough depth you might be able to fit the plate flush with the wall.

I wouldn't chocbox outside the wall - that wouldn't pass code.

This might be one of those jobs that you have to get an electrician for - given the regulations.