Usually when a roundabout has two lanes on and two lanes off it then it usually means that there is two lanes on the roundabout, not one for you to weave around in.
The next person you cut up might not be so quick with their brakes and then that's a big insurance claim and full liability to you.
Check your mirrors and use your brain!
Maybe don't swear at someone you've just cut in front.
Luckily for you I don't care enough about your poor driving ability but maybe someone one day will...
Crap, dangerous driver.