In the same vein ,I once remember seeing in a local RR garage a rolls with the single number 6 on its reg plate ,were there any uk plates ever issued with single numbers ,or were they always letters and numbers ?

The first registration marks were made up of one letter with one number, the letter denoting the issuing authority (the first, A1, being issued by London County Council in 1903.)

Later formats were comprised of two letters and four numbers. These series were replaced as and when they were exhausted. So, whilst the Liverpool series KA lasted for only two years between 1925 and 1927, the series SJ was still being issued by Bute Council in 1963.

By the mid 1930s, the two letter/four number series of registration marks were exhausted in some areas, and new three letter/three number series were introduced. AAA 1 was issued (by Hampshire County Council) in 1934 through to AAA 999 and the commencement of the BAA series in 1936. This process continued until the mid 1950s when marks were reversed to three numbers/three letters.

This site gives a comprehensive account:

tinyurl.com/d8l8eyo