Don't forget Dahons. Unless you have a dislike of derailleur gears, there are several models that could suit you well, and save you an arm and a leg. You could import an '08 Jetstream XP (a good match if not better than the Birdy in many respects, not available in UK sadly), have a Mu SL as a (very) high speed runabout, and still have change compared to the Birdy Rohloff. And the Airnimal Joey has 24" wheels, not 20".
I'd rather spend time exercise by riding on the bike rather than carrying it, personally (only 59 kg myself).
Don't confuse wheel size with folded size- the two do not necessarily match up, and ease of folding is important too.
Rohloff is most definitely an aftermarket-fit option as well.