Thanks for your reply.
I've found the following which gives some indication of the number registered.
Quote:
3.1. Nominet will not register any two letter Domain Name in the .co.uk SLD. However, two character Domain Names are allowed, ie 3x.co.uk.
3.2. Nominet will not register any one character Domain Name in the .co.uk SLD (ie a to z inclusive, 0 to 9 inclusive or hyphen (-)).
Although there is one exception to rule 3.1 that I have found !
I think there are 520 possible combinations with two characters as 3.1