You want Evergreen plastic tubing. They have a contact email to see if they can tell you of a supplier close to your location. Many of the online hobby shops carry Evergreen so check your usual online shop in the UK.
www.evergreenscalemodels.com
Plastruct also has a line, you can check their website to see if a hobby shop carries it.
http://www.plastruct.com/
Basically look around for railroad modeling shops, they often carry this stuff too.