Motorways in England, Scotland and Wales are all given the designation 'M' followed by a number which relates to a predetermined area of the country. For an explanation and a guide to those areas, follow this link;
http://www.netcyclo.com/places/polit/nations/uk/uk -tmw.htm
The numbering of motorways roughly follows the original numbering of the trunk road system. Hence, the M1 parallels the A1, the M6 the A6 and so on. Even the M25 follows the route of the A25, somewhat, to the South of London. This also applies with double or triple digit numbers, so the M56 runs by the A56. Also, the designation (M) - this refers to a previously A road which has been upgraded to motorway status, eg A1(M).