Area code 020 is used for London landlines, which have eight digit local numbers. Mobile phones (07xxx) and special-rate services (03xx and 08xx) use their own country-wide prefixes.
To call from abroad, use the country code +44 and the full UK number minus its leading zero. For example:
Somewhere in the United Kingdom
The country code and area code of Coventry, United Kingdom is 44, (0)24.
The country code and area code of Birmingham, United Kingdom is 44, (0)121.
The country code and area code of Cambridge, United Kingdom is 44, (0)1223.
The country code and area code of Scarborough, United Kingdom is 44, (0)1723.
The country code and area code of Northwich, United Kingdom is 44, (0)1606.
The country code and area code of Aberfeldy, United Kingdom is 44, (0)1887.
The country code and area code of Abergavenny, United Kingdom is 44, (0)1873.
The country code and area code of Aberystwyth, United Kingdom is 44, (0)1970.
The country code and area code of Abingdon, United Kingdom is 44, (0)1235.
The country code and area code of Abington, United Kingdom is 44, (0)1864.
The country code and area code of Aldershot, United Kingdom is 44, (0)1252.