01 was the area code for London.
On 6 May 1990, the 01- code was split into inner London 071- and outer London 081-.
On 16 April 1995 these codes were changed to 0171- and 0181-.
On 22 April 2000 both codes were consolidated into 020-.
An 01-xxx xxxx number became either an 071-xxx xxxx or 081-xxx xxxx number (ditto 0171- and 0181-) and today the original 01-xxx xxxx (if unchanged) will be either 020-7xxx xxxx or 020-8xxx xxxx.
Please note: the correct area code for London is 020-.
0207- is actually 020-7xxx xxxx and 0208- is actually 020-8xxx xxxx