I suspect it’s a combination of factors including low demand, lack of expertise, cost of importation etc. Anything a bit different is always expensive in UK construction. I was recently quoted £5k for a pair of external sun blinds. I ended up driving them over for Germany for less than £1k.

Difference between UK and Germany? In Germany most homes are self-built as homes. In the UK they are developed boxes for max profit. ".. there is a historic issue with honestly and fair pricing (due) to the British seeing property as speculative/an investment as much as seeing property as homes."