Hi IWTGF
I travel on a 20 mile commute into London Paddington myself and then use a folder in central London. No tube (subway) anymore such bliss!
You have quite a good budget and will have a thousand opinions. If I was going at £550 and above I'd go for a 20" folder. they fold compact enough for trains and have a much better ride for all round use. In fact my own folder rides faster from a standing start than my Specialized which is a full size £400 bike. The larger 26" wheel only outperforms the 20" wheel when touring on downhills as the freewheel kicks in and makes a difference to performance bikers.
If you want to stretch a little further I'd recommend the Dahon Mu SL as its over 1kg lighter and will also ride much better.
I would not write off the Downtube on clearance at the moment on eBay at £209 with 8 speed Sturmey Archer gears. OK I know you wanted the support of a UK shop. However I have one of these and I think equals the more expensive Dahon hub bikes that are £550 and upwards. SA 8 speed gears cost £150 on
www.ebay.co.uk just as a standalone component.
As you are a fellow commuter, you are welcome to check my bike in person if you are ever in the same day as myself in London over the next few weeks. I work as a lecturer at a central London uni so I'm fairly flexible.
I don't see any listed at the moment but I'm sure the owner of Downtube still has clearance stock left in the UK and may well do you good deal on delivery as opposed to the customary £49 delivery price. Really the hub gears are unbeatable in the city where you can change back down gears at red lights and lower maintenance too.
For peace of mind there are knowledgeable bike stores can service this bike as and when necessary in London such as the one below:
Velorution (Oxford Circus):
http://www.velorution.biz/?page_id=996