| M | Cheshire |
| MA | Chester |
| MB | Chester |
| MC | London (North-East) ( Middlesex before 1965) |
| MD | London (North-East) (Middlesex before 1965) |
| ME | London (North-East) (Middlesex before 1965) |
| MF | London (North-East) (Middlesex before 1965) |
| MG | London (North-East) (Middlesex before 1965) |
| MH | London (North-East) (Middlesex before 1965) |
| MJ | Luton |
| MK | London (North-East) (Middlesex before 1965) |
| ML | London (North-East) (Middlesex before 1965) |
| MM | London (North-East) (Middlesex before 1965) |
| MN | Isle of Man |
| MO | Reading |
| MP | London (North-East) (Middlesex before 1965) |
| MR | Swindon |
| MS | Edinburgh (Stirling until 1981) |
| MT | London (North-East) (Middlesex before 1965) |
| MU | London (North-East) (Middlesex before 1965) |
| MV | London (South-East) (Middlesex before 1965) |
| MW | Swindon |
| MX | London (South-East) (Middlesex before 1965) |
| MY | London (South-East) (Middlesex before 1965) |