Bike Friday would definitely work, as they do multiple frame sizes and custom jobs. And folders are definitely suitable for various purposes. The challenge with smaller size wheels is getting high enough gears, low ones are fine. My mezzo (16" wheels) has gone to
Brighton several times (but never up Ditchling), I took it on the Dunwich Dynamo a few weeks ago, and cruised passed lots of road bikes up the first hill. I've subsequently bought a road bike, but that's improved my speed by maybe 2kph, and that's all on the flat and mostly because I've got higher gears and feel I should push harder.
People have done Paris-Breast-Paris on Bromptons (1200km in 90 hours). Until a month ago, mine was my only bike for the last 4 years, and since I started counting January 2011, I've done about 12,000km on it, including 52 km commutes.
I get a lot of "you're brave", "do you think you'll make it on that?" when I do group rides, but the truth is if you are comfortable in the cockpit, and you are in an equivalent gear (ie same in inches) to a full sized bike, then you will be doing about the same work on either. Your task is to find one you are comfortable on. I'd try a mezzo too, if you can find one.