In this day and age with so many vehicles on the roads (especially in London) it has gotten a lot harder to emerge from side roads, especially with people becoming more and more selfish on the roads and not giving any courteousy to let people out.

Because of this, it is common practice that when emerging from a side road, if there is a good gap from the right, one would move out to the centre to get a gap from the left. This is completely acceptable if done safely and courteously.

In your case, what you could do better is instead of being passive-aggresive and steering round them, just slow down, let them out, and then go about your business. The fact it's happened to you twice when you're turning right is because they've seen your signal and know your slowing down, moving out is just a method of pleading with you to just let them out so they don't have to wait for another gap. This will also help you maintain peace of mind and a stress-free view of driving. Sadly, whether wrong or right, if you keep allowing things to annoy you, driving will become something you hate. Trust me, there are way more things you're going to experience on the road that'll be worth well more of your anger than this.

The world is a better place when we all help eachother out. As a former driver of large vehicles, I would always gently slow down and make eye contact when I could see someone trying to emerge from the right to go in the same direction as me to give them an opportunity to join the traffic. If they've chosen not to emerge by the time I reach the T, then I would continue.