Just come across this thread while searching for all things Birdy related. I've recently converted a Mk1 Birdy to a Nexus 8 speed IGH. I was finding the chain prone to slipping off the cassette when folding with the previous derailleur as well as it being more prone to picking up dirt (my intention is to convert it to belt drive at some point.

I used a Brompton chain tensioner which, with some judicious fettling, mounts to the back of the derailleur hanger and lines up perfectly with the rear sprocket. I'm tempted to drill the underside the swing arm so I can route the gear cable up the inside. Also to come are some lights to wire into a dynohub up front.

On that note, what are the preferred mounting points for a front light? It feels like options are limited with the fold.

I would post some pics but currently not up to the 10 post threshold