Just a note on the birdie. It's a really good bike off road. Espically as it offer the advantage of bigger tyres, big apples included,better brakes and front suspension.
I own three birdies and the drailler near the ground can be a problem. It's not such a problem on the ihg version, but the cage extention on the drailler version can get jammed back in to the cassette.
There are solutions to this. Recenttly I bought a paffiic chain tensioner. Expensive but negetes the need for the cage or a ccn bolt extention can replace the cage. Don't let this aspect put you off the birdie as there are solutions.
It sounds like a brompton suits you best but don't over look completion. For instance;
Birdie with a front rack is the best bike I have tried to roll. Better than mezzo, brompton etc.