Project Plastics Whitehall industrial estate in Colchester. They made mine for my previous boat whilst I waited for a very reasonable price, but it was 2004
Hadlow Marine (marine window specialists) do, or at least used to do, acrylic washboards, with a wide range of colours, and options for vents, wooden trim and handles, etc. They have a website with a catalogue that contains prices and options. The chap who runs it is very pleasant to deal with.
I used JP from Hadlow for Gladys windows a few years back, and yes he was great. Unfortunately, he has stopped his site visits and installation service because of knee issues... But, for washboards that you almost want done while you wait (take old ones in as templates etc...) he's a long way from North Essex