Bangle.js v2 Developer Edition

but I had to pay customs extra as the UK is no longer part of the EU...

I know, sorry about this (although we don't add VAT any more so the actual extra charge is probably around €6). We are trying to find a way around this but unfortunately it's not easy.

is there a dedicated forum page for Bangle.js v2, perhaps even one for the Developer Edition

There isn't actually - maybe there should be, but I'd have thought for now the Bangle.js 1 forum would be ok.

in case that there are some differences between Kickstarter and Developer edition?

There shouldn't be... About the only difference is the lack of water resistance on the developer unit (you could glue up the pressure sensor hole if it's a problem) and that the charge cable isn't short circuit protected (although the chance of having a short is much less with these cables anyway)

do I still have to care about the orientation of the magnetic charging plug

Yes and no. It only clips on one way around, but it doesn't matter if it touches the watch the wrong way around (it didn't matter on Bangle.js 1 either)

how long does it take for the device to get fully charged?

I haven't timed it, but definitely under 4 hours. It'd be well worth upgrading to a recent cutting edge build of the firmware, as power usage has dropped by at least 4 from the version that's shipping on your unit.