Firstly, there's no such crime as "hit and run" in UK law
There's 3 different cases, all covered by Section 170 of the Road traffic act:
a) Failure to stop at accident and/or give details (name or address)
b) Failure to report accident, if they did not/could not stop/give details at the scene (e.g. they crashed into a parked car, and the owner wasn't around)
c) Failure to give insurance details (within 7 days)
There are a few caveats however.
If the crash only involves your vehicle or property, you do not need to report it.
But also, it's not just about cars: Crashing into any property you don't own needs to be reported
However, on an interesting aside, when it comes to hitting animals there's a specific list.
You have to stop with horse, cattle, ass, mule, sheep, pig, goat or dog, but nothing else. (Not that you can hit cats but not dogs!)
Back to the original post.
Section 170 does not take into account who caused the crash, just that the vehicle was involved in the accident therefore, even in a simple rear end smash both parties have to stop.
Furthermore, by "involved" the vehicle doesn't even need to have touched any other vehicle, just that it was a part of the incident (so you cutting someone up, causing them to break and loose control would still mean you have to stop)
HOWEVER
All of that comes with the caveat that the driver was aware of the accident, or at least reasonably should have been.
In the case of a car hitting the rear end of a truck, it's not unreasonable that the truck driver wouldn't have actually been aware of the accident taking place, so he has a defense for not stopping per part a) and b) of the three offenses above.
For the person who hit him, if they fail to report it they can be convicted under part b), as even though the other drive drove away the accident still should've been reported