Hi,
I live in Clapham and have recently had mine replaced by a company called Alaskan. They specialise in sash windows in these areas too I believe!
https://www.london-sash-windows.com/sas ... es-london/
I had to have my original wooden ones replaced as they started rotting. It wasn't the cheapest on the market, but the quality is amazing and the guys that fitted them were pleasant

Its great to have a warm home again
I once had a door fitted from a different company because it was the cheapest option and it didn't close evenly so it's worth the extra pennies for something that'll last a loooong time
I hope this helps!
Gracey x