In the video of the whole incident posted above, the ambulance is the yellow and green van which is behind him at first but carries on to the left out of our view. The two white vans which follow him to his crash site are probably police vehicles.
I suspect he had some kind of attack in mind, got spooked by the blue lights and sirens and went for an unplanned blitz attack. He didn't intend to attack just then as he was initially heading the wrong way. It's hard to tell due to the low resolution but one or both of the police vans might also have had its blue lights on and it could be that rather than the ambulance which spooked him into thinking they were chasing him (when they quite likely were not).
Could be. Seems his own car is white and 'very old', whereas this one is silver and 8 years old. It also seems he was in London to renew a visa, yet the event took place at 7:30 a.m. after he'd been driving in London (iirc) for some 90 minutes. It's pretty hard, even in London, to drive purposefully around for 90 minutes and find yourself in Parliament Sq at 7:30
It's almost pure speculation, but it looks like a wildly unplanned and botched attempt at an attack.