Thank you guys for all the replies and usefull information!
I would fly with a client but he didn't make his mind up yet when and what places he wants to visit so I'm waiting for that.
I will take a closer look to the airfields and connections you suggest me and as soon as I have more details I will let you know, but I understand that anyway for GA it's not that simple to get really quick into central London, expect when I land in EGML? Ofcourse i'm looking for the cheapest solution :-)
I fly with a P28A so short runway/grass isn't a problem for me (if wx ok), good way to practise the short field take-off and landings again
Thanks again guys