There's an overheard camera shot, rather unclear, but it looks like the driver took sudden evasive action, came off of one road, across pavement / grass & then rammed the barrier. I can't be sure, because the picture quality isn't good, but it looks like he might have been being followed by a police van. It may be that the sudden evasion was because of that, rather than any act of terrorism.

There'll be more details in a few hours; not much point in speculating now!