Who let's you through
My experience in the south east is that Luton are the most friendly of people and will accomodate wherever they can - although they don't do a LARS anymore, Thames are pretty accomodating, I've been through their overhead via Canary Wharf at 2400 a time or two. Had to refuse being sent over London Bridge in a PA28 once though.
Solent (as has already been said) are fine. Farnborough provide the FIS for most of the area from Fairoaks south and west and always let you through their patch, Odiham, Blackbushe keep a sharp lookout at Lasham etc.
The only people I have never ever got a passage from are Essex Radar (Stansted) even one day offered to take an IFR and go over the top but nothing doing.
There is a long post somewhere about Heathrow (revitalised on the GA site about 3 weeks ago) (with a lot of good posts from the LHR ATC guys) who will allow corners to be cut from Fairoaks via Ascot and Burnham (subject to which way the wind's blowing).
My early days at Manchester (flying at Barton) they used to do a RIS (edit: it was more likely FIS) down the low level corridor but I believe these days it's best not to even call them. just keep in the corridor (assumedly with a 7000 squawk) and keep 1250 on QNH.
That's the limit of what I can remember at the minute.
DGG