Local Suppliers:

http://www.lasercutouts.co.uk/ -  laser ply, acrylic and high density board. (Warning: This web site contains Comic Sans)

Based in Washington. They are a Laser cutting service, with a few machines similar to ours and 2 bigger machines with 5 foot square beds. Main business is cutting non metallic materials for craft supplies, but are willing to supply materials to us. open 7 days a week except for the run up to Christmas and between Christmas and new year. Normally has 3mm laser ply ready cut to 600x400 at about £3 a sheet as this is what they use. They will give us a quote if we want to make a bulk purchase. They cut larger sheets down to size and some may have small amounts of damage from the saw and the usual knot patches. Send them an email for price and availability.

http://www.bayplastics.co.uk/ - Acrylic

Based in North Shields. Just about any size and type available from the trade counter. Open 8am to 5pm weekdays, and 8am to midday Saturday. A bed sized 3mm perspex sheet comes in at about £9. 

http://www.lathamtimber.co.uk/

James Latham - Felling Industrial Estate.

They are a timber yard who supply Laser Ply.  They sell 3mm laser ply in 5'x5' sheets (Other thicknesses/sizes to be confirmed).  They do not provide a cutting service but the 3mm material will flex enough to fit in a car.

Online Suppliers:

https://www.kitronik.co.uk/laser-compatible-plywood-and-mdf.html 

Flat delivery charge of £6.50 per order.