At 231 miles (370km), the M6 is the UK's longest motorway and extends from Catthorpe (junction 19 on the M1) to the Scottish Border (Guards Mill) where it becomes the A74(M) which itself joins the M74 to Glasgow. Taken together with the A74(M) and M74 (which is itself being extended by 5 miles) the total continuous length of motorway will be about 370 miles. It is very unlikely that the sections in Scotland will be renumbered.
The first section was opened in December 1958 (Preston By-pass) and the last exactly 50 years later in December 2008 (Carlisle to Guards Mill). It is generally dual three-lane carriageways, with short sections of dual four-lane between the M56 and M62 and between the M61 and M55. The section North of Birmingham is so seriously congested, that a new motorway, the M6 Toll, Birmingham Northern Relief Road opened in December 2003 to provide an alternative route.
The Highways Agency website suggests that the M1 is the longest motorway, but since it is about 41 miles shorter than the M6, this must be an error. Even when work on upgrading the A1 to motorway standard is complete, the total continuous length of motorway will only be some 215 miles.