Many years ago - back in the dim distant past of the 1970's!!!
It was alleged that there was a mini driving around Central London with Fiona Richmond at the wheel (Paul Raymond's girlfriend/mistress) and bearing the registration number FU2. Currently showing on DVLA as 'Not Found', so this may be an urban myth.
I also remember from the 1970's an article in the national papers about a registration plate that had turned up. It was supposed to be 505 EXY, but a little playing with the spacing came out as SO SEXY. According to DVLA this number is currently on a 2019 Land Rover.