Andrews and Arnold offer this service, I have used them in the past and they were a good isp see http://www.aaisp.net.uk/aa/adsl/multiline.html
Thanks Adam
To those considering A&A..
I've had clients that have used A&A, and in dealing with them, they know their onions and support bsd and linux and stuff in the way an isp *should* - ie, if there's a problem they'll actually help you solve it, and if you're/they're stumped, they give constructed educated help instead of saying "reformat and reinstall, it's all your fault".
Had very good experiences with them, and can strongly recommend them to anyone who's considering them.
(I thought I'd offer this input since other people's experiences with an ISP can be a consideration :) )