In message ,
Martin Underwood writes

By the way, how did changing from 0171 xxx yyyy or 0181 xxx yyyy to 020 7xxx
yyyy or 020 8xxx yyyy help alleviate the shortage of available numbers in
London? It didn't increase the number of available phone numbers - all it
did was to change the mapping slightly. OK, so there's scope for additional
district codes beginning with digits other than 7 or 8, but it's not
districts that are in short supply, it's subscriber numbers (the xxxx in the
above example).

Just to add to John's reply, London numbers starting with 020 3xxx are
due to start being allocated this summer. Unlike 020 7xxx and 020 8xxx,
they will be assigned on a London-wide basis and will not be mapped to
any particular district within London.
--
Paul Terry