The initial 'digit' was actually printed as a capital O during the days pf Alpha/Numberic codes - see O270 on the attached.
It didn't really matter as the 'O' and '0' were in the same location on the dial - the tenth digit - unlike the American/Canadian dial which has the letter 'O' adjacent to the M and N on the digit 6. Note you'll occasionally come across GPO type Dial No 10/12's with the letter 'O' on the digit 6. These have originated from UK manufactured telephones supplied to Canada.
Once All Figure Numbering started to come in 1966, the initial 'digit' was printed as a 'zero' but folk carried on referring to it as 'Oh'
Modern mobile phones (and the odd modern 'landline' phones with letters) of course follow the US pattern of lettering
