Format        ; Parent Format ; Comment
20X XXX XXXX  ; fixed_2/4/4   ; "Historical London format before it moved to a two-digit area code"
20X XXXX XXX  ; fixed_2/4/4   ; "Historical London format before it moved to a two-digit area code"
20 XXXX XX XX ; fixed_2/4/4   ; "Commonly used London format"
XXXX XXX XXX  ; fixed_4/5-6
XXXX XXX XXX  ; mobile_4/6
XXX XXXX XXX  ; mobile_4/6
XXX XXXX XXX  ; fixed_3/3/4
XXX XXX XXX   ; tollfree_3/6
3XXX XXX XXX  ; fixed_3/3/4   ; "UAN numbers beginning with 3 are also often formatted with a 4-digit first group (many search results)"