Problems updating my One M8 from 4.4.3 to 5.0

In december 2014 I got my HTC One M8 from amazon.de. (Note that I live in germany) I purchased the Phone without any SIM lock. While it is a good phone I have not received any update for android yet. The Phone's stock version is android 4.4.3. A coworker who has the same phone than me was able to update to 4.4.4 and 5.0 a few days back.

When I go to Settings > Info > Software Updates it says that the phone is up to date and it stays at 4.4.3

I've contacted the HTC Support and according to the serial number my phone seems to be from the UK and that is why the phone doesn't recognize any new updates.

So my questions are:

Why does that even make a difference? And how can I update my phone to 5.0?

Thanks in advance