Re: @Brian 3 - Sorry, mate
Thanks for getting back, Chris. Which leaves only one option based on the information provided - the car matched the speed of the lorry all by itself...
If that was the case, then it's at THIS point in the drive where it should have been obvious the car didn't just have standard cruise control."
I think that is exactly it - the cruise control was set at a point it wasn't able to attain 70mph. For whatever reason (and I'm not the OP), it became obvious that it was adaptive only when the acceleration began automatically. The OP didn't say that s/he had a problem with it, just that an unwary driver could have been caught out, and ended up in a situation where the car ran into something.
For my part, upon re-reading what the OP wrote, I can see that there are some oddities in how it is constructed, but the message still seems to be clear to me - it accelerated without driver input, which is (to my mind) inherently dangerous, especially so if not ready for it.
When I have a hire car, I don't play with the toys until I've got used to the basics (around 30 miles, usually), and definitely not in traffic. If possible, it'll wait until I've read the manual or found information online. I don't usually drive modern cars, and so know that there will be some hideous piece of automation that will work contrary to my expectations of being in control.