| X | Northumberland |
| XA | London. (Kirkaldy 1964–74 with year suffix) |
| XB | London (Coatbridge 1964–1974 with year suffix) |
| XC | London (Solihull 1964–1974 with year suffix) |
| XD | London (Luton 1964–1974 with year suffix) |
| XE | London (Luton 1964–1974 with year suffix) |
| XF | London (Torbay 1964–1974 with year suffix) |
| XG | Middlesbrough (until 1974), subsequently unused |
| XH | London |
| XJ | Manchester (until 1974), subsequently unused |
| XK | London |
| XL | London |
| XM | London |
| XN | London |
| XO | London |
| XP | London, later temporary plates for vehicles being exported to Europe |
| XR | London |
| XS | Paisley (until 1974), subsequently unused |
| XT | London |
| XU | London |
| XV | London |
| XW | London |
| XX | London |
| XY | London |