If what you are saying is correct (that residents of Highley cannot receive DSL) then this would certainly appear to be a false claim. Before, contacting the Advertising Standards Authority (which is, I think, the relevant organisation for press and internet advertising as well as TV/radio), contact BT first to ask if DSL is available in the area in question. If it isn't, then perhaps there is a case to be answered. What is the exact nature of this website? i.e. who is responsible for it? Does BT have any association with it? Is BT's logo, name, etc. used with or without its knowledge and approval? It may not be deliberate on the part of BT, or indeed be BT's fault at all.
Find out more about this site, and then contact the ASA if you still have cause for concern. If someone (BT or otherwise) is making a false claim, then the ASA will be interested.