for roll film I usually use http://www.speedgraphic.co.uk/ cos they are cheap but it looks like they only have tri-x in 120.

you might try phoning firstcall but again its only listed in 120 http://www.firstcall-photographic.co.uk/

both have much lower minimum order values but you have to pay shipping. Firstcall are very helpful. Speedgraphic are just cheap and don't cater for the specialist whereas firstcall are predominantly for film users.