Giok & C-Flat,

"PEN 15" was certainly around in the '70s, it was given by Paul Raymond (of Raymond's Review Bar) to Fiona Richmond (anyone remember her?), but there were restrictions placed on it, at the time the (then) equivalent of the DVLA[1], would only allow the number if the the plate was on two lines.

I seem to recall that Bob Danvers-Walker (Radio Announcer) had the number plate "RAD 10"
Jimmy Tarbuck had "COM 1 C"

As has been mentioned, British number plates have a set layout. I do recall one "OHP 150 F" which, with the addition of black screws to hold the plate on became
"OH.P15.OF"

Walrus