Cormac,
Off the top of my head, the following 'London' airfields and operators have S300s:
Biggin Hill Helicopters, out of Biggin Hill (funnily enough!) Not sure of their current status with regards to training though. A quick phone call to the airfield should be able to confirm this for you.
Cabair have recently bought 2 S300cbis. I think that one is based down in Rochester. However, they might be willing to reposition it to one of their other bases for you. They are based at Elstree, Denham, Blackbushe and Biggin (amongst others.) A phonecall to them will obviously confirm this. (I've just had a quick look at their website and it isn't initially obvious which bases cater for PPL(H) training.)
Helicopter Services (Wycombe Air Park) might be able to source an S300 for you for training. Once again, you'll need to phone to confirm.
HTH
Edited to add: Ah, just noticed that I've cross-posted with VNE.