Try 3D displays - UK based acrylic manufacturer, MDP - UK based, those are two who make/cast acrylic/resin plinths. There are many other UK suppliers who make wooden plinths who are members on here. I can't think why you havent come across any indigenous makers/suppliers
Oakwood Studios, UK based, fair priced and they make to your specifications. And Mr Wharton is a gentleman! http://www.oakwood-studios.co.uk/
I've been a mail order customer for ages, even if Royal mail rates are gigantic.
Pierre
Hahaha... bet that's been pointed out to him many times too .
Those Darkmessiah plinths are very original as he's using aluminium masters for smooth moulds, and he's casting in black resin for a consistent colour.