If you're the one changing lanes and he's already in that lane and approaching from behind, you should give way to him and let him past first. Pulling out in front of someone and causing them to have to brake is no better than pulling out from a minor to a major road at a junction and causing someone to have to brake to stop from hitting you.
This. If someone pulls out way ahead and I can see it from a mile off I just coast until I'm behind them. But when people pull out right in front of me and I have to brake, I'm none too pleased. The worst is when people pull out of side roads in front of you, only to then turn off 100m down the road (usually without indicating), causing me to brake twice. FFUUUUU
As for the motorways, I find weekend drivers are the worst. I swear they never use their mirrors. And then they're the ones giving you evils when you finally get passed
I find the problem here in Belgium is that if you keep a safe distance on the outside lane on a motorway, one of three things will happen:
1. Someone will take that space, thereby slowing you down and negating the safe distance.
2. The person behind you gets irate because you're not going fast enough (as in you're too far away from the car in front).
3. The car in front will never pull over, as you're too far away.
Over here you really have to drive up their chuff to get them to move over, making tailgating almost necessary to make progress on some stretches of road

I only ever flash to get them to move if they're really taking their time over it, or they're hogging the middle lane. But swerving across 3 lanes usually gets them to move..
The only time I overtake on the inside is during flowing traffic on dual carriageways where everybody seems to prefer the outside lane, and on the very, very rare occassion that they don't ever move.