Some of the exchanges were given new prefixes when the letters were dropped, and some were changed in later years, but quite a few of the original codes are still in use.
The situation is a little more complex in recent years due to the numbering plan changing. First, the original 01 London area code was split into 071 for inner London and 081 for outer London in order to double up the capacity. More recently London was put back into a single area code (020) but a digit 7 or 8 added to the old number to make the local numbers up to 8 digits long.
You can still see the original prefixes in many cases, however, e.g. the GERard exchange still serves the same area around Soho, only the numbers are now listed as 7437-xxxx, and the old ENTerprise prefix lives on in New Southgate as 8368-xxxx.
