Send an enquiry to them at http://business.ntl.com/static/customer_support_form.php it dosent mention IRC Servers on there terms of use, Web Hosts put it theres because its a continunual process and akes alot of ram up on theer server.
But you want go over you download limit by running an IRC server off it, i once got a warning for going over my 20BG Download limit so i upgraded, Your ISP seriously wouldnt have a problem with this why dont you email them
Its very unlikly that it would take much, up ive been running Paper IRC on and off for about a year and i have 30BG Download limit and my ISP has never had any problems with it and if its running on you computer it shouldnt cause any problems.