The VAT rate should be 5%. (Source: HMRC Notice 701/19, August 2012: s2.1, on fuel for domestic use, and s3.2.2, defining "domestic use" as including use in "houses, flats or other dwellings".)
Both companies insist on charging 20%.
I've offered to give them written confirmation that I am buying the wood for domestic use as fuel, but they aren't interested. They both say that they won't sell wood to anyone, domestic customer or otherwise, except if they can charge VAT at 20%.
If I did pay four times as much VAT as I should, I strongly doubt that I would be able to get my money back from the HMRC.
If they do get someone to pay 20% who should only pay 5% (and I imagine they have done that in loads of cases), is that VAT fraud? Or just a tort against the customer?
(PS I've only recently encountered the world of buying firewood, and there seem to be loads of sharp practices, but I digress.)