Bikes were the first rubber wheeled vehicles on the road, they're even the reason why paved roads were first layed. I've been pulled over by police while riding my bike here in Canada. They straight up told me that the same rules apply for bikes as they do for cars. Those include speed limits and traffic laws.
I don't know UK laws, but I wouldn't expect to get anything out of rear ending a vehicle on my bike, because they would have had the right of way, if the same situation had happened here, I would be considered to be the party at fault, and therefore would be liable for the damages.
As far as the speed claim, as long as you were doing the posted speed limit, then they shouldn't be able to touch you with that one, unless there's a different limit for cyclists in the UK. In response to their claim that you may have been exceeding whatever limit they claim you were, all you have to say is, "Prove it." They will then be required to launch an investigation to determine how fast your were going based on impact forces, which is determined by examining the damage sustained by the vehicle, and you.
In your situation though, I'd consider myself extremely luck if I got anything out of this, because like I said, when you get into an accident on your bike on the road, they treat you like another vehicle, so you need to think about the whole situation as if you had been in a car. Had you been going the same speed and been in the same situation while driving a car, then what would you expect the outcome to be?
Edit: After reading the comments and learning more about the pileup being caused by an oncoming vehicle, the only thing I think you could possibly be found guilty of is not maintaining a safe following distance. Obviously the oncoming vehicle is at fault, you should feel lucky that the car overtaking you softened the blow, had you been the one that hit the other car head on, chances are you wouldn't have woken up in the hospital at all.