Since 1921, Great Britain has issued special registration plates with a “Q” prefix.  These designate cars that are imported into the UK from another country.

The Q plates are issued to 1) someone who has lived outside the country for more than two years and is entitled to import a car tax-free, 2) the vehicle is unregistered in its country of origin or the overseas registration has expired or the plates must be returned in the home country because of insurance or other legal  requirements, 3) the plates expire automatically when the vehicle leaves the country of registration, and 4) the registration on plates is not understandable to local law enforcement, such as plates from countries with another language or script, like Arabic. 

In this case, registration plate #QE 2044 was issued between 1951 and 1956.  They appear to have been used on a car imported from Mexico, perhaps someone assigned to a diplomatic post in London.  

The plates have a familiar “GB” oval affixed to the front long plate.  Both plates have a professionally prepared set of attachments with “MEXICO” stencilled and an embossed border.