10mm on a 40A fuse would be my method. Unless there will be an electric shower and electric oven / hob. You can get 13A induction hobs. Is this a separate annex? In which case perhaps 16mm on a 60A fuse. But 25mm on a 100A fuse, that is good for a big detached house.
Using the TLC online calculator for 3 core SWA, 20 metres long and for a lighting circuit: 15 kw >>>> 10mm 20 kw >>>> 16mm (and up to 30m) With a shower and cooker your could see 15kw loads, but 20kw - very unlikely.
Well I can tell you now your gonna make one hell of a lot more mess trying to run 25mm than 10mm or maybe even 6mm. I would double check, 25mm don't sound right me.
I noted the branded cable also does not comply with BSEN numbers not needing RCD protection, that and concentric may do the job but neither comply. However when I found I needed 240 mm sq I used two 150 mm sq instead to be able to bend them around corners. So likely 2 x 10 mm sq would do the job. As to why 25 mm sq that does seem strange, the supply to a house never exceeds 125A in fact more like 100A so 25 mm sq does seem to be over kill.
What is you main incoming fuse rated at? They can quite often be 80 amp even if holder says 100 amp. You will need segregation between the fuses. 100 amp fuse may be to big As your spark done diversity calculation to see what the loading will be? Sometimes it’s better to take a slightly longer cable route to get it In neater and hidden. We usually find a way of hiding cables or using surface trunking in corners or behind furniture. In corners and caulked in it disappears.
It's an 80A fuse, even though - like you say - the holder is marked as 100A. So if he agrees 25mm is overkill, would t&e do? If it's just under floorboards and not going outside, I'm guessing that would be better?
DO NOT use twin n skin. You will almost definitely have to install an RCD which you don't want at the start of the sub-main and anyway the cost saving will be tiny. Tell us what electricals you want in your loft conversion and we can advise what you need.
They also run it up walls and through holes to the customers service head. I considered that burying it posed a greater risk of damage than clipping direct.