Well, that's fairly typical of the poor standards set by many U.K drivers. For a start, the truck driver was not leaving enough space between himself and the vehicle in front. There should be a 2 seconds gap, I doubt that that gap was even a second. Sadly something like 90% of drivers don't leave enough space. The truck driver could easily have slowed down a tad to let the other driver tuck in but TBH he probably wasn't expecting such a stupid move. As for the civic driver, what a wally.....you should never attempt to undertake while you're filtering in, just back off and be patient. A good driver would realise that trucks have blind spots and also the Civic crossed the chevrons because he'd run out of space.
Pete