I used Kelly's Blinds https://www.kellysblinds.co.uk/ for these. Felt surprisingly cheap ( ~ £100 each), they came out and measured up and installed them, all custom sizes. Knew their stuff and gave some useful recommendations.
Yes, the windows are pretty ugly PVC and getting blinds that fit them exactly make them look a lot better. There was a whole variety of slat and tape options to pick from.
I used Kelly's Blinds https://www.kellysblinds.co.uk/ for these. Felt surprisingly cheap ( ~ £100 each), they came out and measured up and installed them, all custom sizes. Knew their stuff and gave some useful recommendations.