I've had three aircraft interiors (seats, carpets & some upholstered trim panels) done locally by an automotive trimming company. Ran it by my maint.org. first to ensure paperwork would all be in order. We're a long way from Odiham, but I'm sure you'll be able to find something similar locally.
For the plastic trim parts, we couldn't find anything in the UK and used Vantage Plane Plastics in the US
Plane Plastics - Airplane and Helicopter Interior Plastic Parts
I've ordered from them a few times over recent years with no problems, they ship quite quickly, about a week last time. You do need to do minor cutting and finishing on some parts. Also be aware, you'll need to pay import duty and VAT, usually to whoever the shipping agent is (UPS/FedEx etc). Remember as well to make sure you get 8130's for the parts.