S&S bikes have been mentioned, but they aren't really folders they're simply bikes that are relatively easy to disassemble for travel whilst still offering a full sized bicycle and the stiffness and qualities that gives.

There are some small bikes along the Japanese "minivelo" style that require only minimal disassembly. Such as the Moulton TSR which is a fantastic bike with a ride quality not far from a full bike.

The Moulton TSR can be found over here: http://www.moultonbicycles.co.uk/models/TSR30.html

However disassembly != folding. For folding I would probably still go with a Brompton first and foremost: http://www.brompton.co.uk/

And then after that a Bike Friday which has already been mentioned: http://www.bikefriday.com/

I would steer clear of the BSOs (Bike Shaped Object : http://www.outrider.org.uk/wiki/Bike_Shaped_Object ) that offer a folding bike for far too cheap a price. There's a reason those things ride like hell and don't have a good reputation.

Oh, and for credentials, I run this thing and am an avid cyclist: http://www.londonfgss.com/