I bought some from B&M Latex Sales Limited about 8 years ago, very firm and charcoal grey just the colour for a gun box. They have since expanded and moved but still in Addlestone.
Copied from their website:-
B&M Latex Sales Limited can be contacted by post, email or by telephone.
Contact By Post
B&M Latex Sales Limited
Inc. Angelic Interiors
Unit 8 Waterside Trading Estate
Hamm Moor Lane
Addlestone
KT15 2SN
By Email
To contact Angelic Interiors by email, please email us at;
sales@bandmlatexupholstery.co.uk
By Telephone
Call our team anytime between 9am - 5pm
Monday - Saturday
Telephone: 01932 847793 or 01932 844040
Fax: 01932 828955
We are open to the general public 6 days and we have plenty of FREE PARKING right outside our door!