There have been dateless number plates that include a Zero as the first preceding number to another number.
There are some pre-1921 "heavy motor cars" (normally traction engines) with lead zeros, e.g plates in AH 0123 format. (source '1903aat').
Some trade plates also have a zero as the first number.
Also KHW 004 definitely exists, however - we're not sure how it came onto the DVLA computer but it's there. (source '1903aat').
Rumour had it that one specially issued dateless plate that included a Zero as the first preceding number to another number was the registration EUR 01.
A database search has revealed that EUR 01 does(/did) exist, but it will take a photo to prove it! So please, if you have one, send it in, as it would make a great addition to this page!
Conformation of a diplomatic registration with preceding zeros came in 2016 with the first sighting of the Royal Embassy of Cambodia's REC 001.
Of course, since the introduction of the current style system of registrations, millions of number plates have now been issued with a Zero as the leading number. Although these '02' '03' '04' '05' '06' '07' '08' '09' registrations are all the 'year identifiers' on 'dated plates'.