Go back far enough at London was 01 and the rest 02-09. London, Birmingham, Manchester and a few others were 7 digits (041 xxx xxxx for Glasgow)

Then London changed to 081/071, then all changed to 01xxx (eg 0564 to 01564, 081 to 0181), then finally London, Southampton, Belfast and a few others mixed to 02x and 8 digits.

03 became national geographic numbers and things like 0345 and 0500 were phased out, 0800 remained free but not always with mobiles, 0845 was “local” but was basically premium, 0870 was even more, 0898 was super premium etc

But as phones took off in the 00s everyone just had 07 with 9 digits. Not sure when that will fill up, but it feels like a billion numbers is enough for now.