I think the HID headlights have a flat beam anyway, so don't need any changes.
The garage will confirm that when they have it on their beam setter though.
Well, it seems that I'm out of luck for the time being.
The beam pattern isn't flat, and I've been told that there's no way that I can pass the MOT with this.
The behaviour of the dipped beam headlights can be overriden manually. You can set their aiming level AND even alter the beam pattern by adjusting a tiny lever located beneath the bulb. Sadly, none of those manual controls are present in the native Bi-Xenon model. The only alternative would be changing the whole headlamps which is insane and I won't do such thing because there MUST be a way to enforce the modifications, I can't believe that there isn't.
So, as a cheap first try, I bought the eurolite stickers from Halfords, which are allegedly compatible with my headlamps as explicitly claimed in their documentation (they're the original Bi-Xenon ones, and the car was built in 2008), but they don't seem to work at all. If I manage to place them to flatten the beam, which seems possible actually, then the amount of light that they throw out is just ridiculous, really dangerous and it wouldn't pass the MOT either.
I just called Alfa Romeo UK this morning and they've asked me for the chassis number of the car to look if there's a way to electronically reprogram the behaviour of the headlamps. I'm working now so I can't look for it but as soon as I can I will be calling them back and hope for the best. Fingers crossed!