Hmmm, I'm seeing this in a slightly different way from everyone else. I'm thinking 50/50.
When the LR pulls out (5 seconds on vid) you appear to be still a fair distance behind (hard to really gauge on vid but maybe 100 yards?). He then overtakes the bus which is travelling really slowly with hazard lights flashing- this may have alerted LR driver to a potential hazard on the road ahead making him accelerate more slowly than expected. You seem to continue travelling towards him for maybe 5/6 seconds at same speed before braking sharply and flashing headlights. This may have startled the driver because of the incredible brightness of your led's causing him to brake. He then pulls over to the left to let you past but you have already moved over to attempt 'undertake' manoeuvre.
So, personally I'm giving the LR driver the benefit of the doubt here.
Agree he probably needs a paint thickness gauge though.
Agree with this, hey we all make mistakes. Does it really matter? Don't let it spoil your day.