It forms an approximate circle spreading 20 miles from the centre of London.
This is deliberate, for historical reasons.
But London is not circular.
So some places outside London have 020 dialling codes.
And some of Outer London is not in the 020 area.
Let's investigate.
Places outside London with an 020 dialling code
There are six 020 telephone exchanges outside London.
They are Bushey Heath, Elstree, Hainault, Ewell, Loughton and Thames Ditton
The most populous areas outside London with an 020 dialling code are South Oxhey/Bushey/Borehamwood and Buckhurst Hill/Loughton/Chigwell, and south of the river Molesey/Thames Ditton/Hinchley Wood and Ewell/Stoneleigh.
Parts of Sewardstone in Essex have a London postcode (E4) and a London dialling code (020). For example Scout HQ at Gilwell Park has postcode E4 7QW and telephone number 020 8138 0191.
Havering is the least 020 London borough, followed by Bromley, Bexley and Hillingdon.
I have spent 15 minutes looking for an 020 phone number in Havering but I have not found one.
Places in Greater London not in the 020 area
19 telephone exchanges inside London don't have an 020 dialling code. They are...
01322: Crayford, Erith, Slade Green 01689: Orpington, Farnborough, Lodge Hill 01959: Biggin Hill 01737: Downland 01895: West Drayton, Denham, Uxbridge, Harefield, Ruislip 01923: Northwood 01708: Romford, Ingrebourne, Hornchurch, Upminster, Rainham
That's seven non-020 dialling codes that definitely stretch into London.
They're the yellow codes on this map.
But I believe eight other non-020 dialling codes can be found in London.
They're in green.
01883: Caterham [for example The Fox, Coulsdon Common - 01883 340737] 01372: Esher [for example The Star, Malden Rushett - 01372 842416] 01932: Weybridge [for example Kempton Steam Museum - 01932 765328] 01784: Staines [for example The Orchard Cafe, Staines Road - 01784 258795] 01753: Slough [for example Airpets, Spout Lane North - 01753 685571] 01707: Welwyn Garden City [for example New Cottage Farm, The Ridgeway - 01707 655026] 01992: Waltham Cross [for example Lea Valley Academy, Bullsmoor Lane - 01992 763666] 01277: Brentwood [for example RJ Skip Hire, Grove Farm - 01277 350775]
It took me a while to a while to locate some of these, indeed some are seriously borderline.
A few are strongly correlated to peripheral postcodes that barely enter London.
Greater London thus has 16 geographic dialling codes
