rating 2.5mm2 cable on timber

A door is slightly better than a double glazed window, but not much.

Neither are a patch ion 3" of celotex.

From the Regs: Table 4A2 (single or multicore cable on a wooden or masonry wall)... 'Cable mounted on a wooden wall so that the gap between the cable and the surface is less than 0.3 times the cable diameter....'

Whilst that is correct for cables clipped direct to a wooden wall, the reference method that the OP has to use is B as he intends to use conduit for some of the run.

You right it does not. Although method A does mention thermal walls and conduit. I might tweak that article later to make it clearer.

It is worth noting that the wiki article is not a replacement for the regs (due to copyright) and it only shows a limited number of installation methods.

If you have any other question on that wiki article please say as constructive critism of these articles is a good way to improve them. You valid point has been noted.

As to the answer to your question then you need not worry. Even if timber was a superb insulator then you would at worst have to use reference method

If some of your T&E is through conduit then use ref method B, again still rated well above 20A.

And as Alan ponted out, clipped direct to timber is method C.

Yes, of course, I didnt read the 'conduit' bit. Ta Alan.

"Clipped direct" (i.e. C) applies for the surface run section.

B assumes the wall material is not an insulator.

Its not a particularly good insulator, however the main factor here is that most of the cable is in free space save for the side touching whatever it is clipped to.