Why does the M23 start at Junction 7?

The thought will have crossed most drivers' minds at some point if they are regular users of the M23. Why, unlike most motorways, doesn't it have a Junction 1 - or for that matter a Junction 2, 3, 4, 5 or 6 either? The reason that its northern-most junction - where it meets the M25 - is Junction 7 will be a mystery to most motorists who drive on it every day.It is all because the motorway was originally meant to stretch deep into south London and start in Stretham. Plenty of people who get stuck