Have been doing some work and research and found the official RAF Battle of Britain page:
http://www.raf.mod.uk/bob1940/stations.html
on it I found the following airfields included in CoD were not part of 11 Group which was responsible for most of the geographic area CoD map covers:
Brooklands
Canterbury
Fainborough
Feltham
Gatwick
Hamble
Heathrow
Heston
Littlestone
Maidstone
Oldham
Portsmouth
Ramsgate
Reading
Redhill
Rochester
Shoreham
White Waltman
Wilmington
Worthydown
And the following airfields that were part of 11 Group are not included in CoD:
Debden
Detling
Martlesham
Rochford (most probably is your Southend)
Stapleford
West Malling (most probably is your Maidstone)
Could this be taken as a suggestion for future versions, since some of them played an important role during the battle.