Jacko - it is in London & South East 1, not just London - lol.
In the same way North, Midlands and South West cover vast tracks of land, London itself is but a small central part of the Division. London itself is not a County, but made up of much of Middlesex - NW of river; some of Herts north of river; some of Essex NE of river; some Surrey - sarf west of river; some of Kent - SE of river).
Plus L= &SE has rest of Essex, Kent, Surrey, and Herts plus Hampshire (Portsmouth) and Eastern Counties (Suffolk and Norfolk [North Walsham]).
That is where the clubs come from, but Hampshire is in South West for referees (with Berkshire, Bucks and Oxfordshire) and I believe Herts is in Midlands (ex-Lucy ?) as far as referees go !
As for the clubs, North Walsham have always been a serious club at this level and higher in National Leagues, but seem to struggle at moment.
Portsmouth were a mid London & SE leagues club for years but invested in coaches and players with new clubhouse a few years ago, got up to London & SE 1, hang in there and now are mid-table.
Journey to North Walsham from Portsmouth is a nightmare (over 5 hours usually), so Portsmouth could be well below par (and some players may not make the trip). Basingstoke and Havant hated the trip in the past and robust agricultural nature of the play.
Good luck and enjoy