Let's play devil's advocate re your theory that MI5 (or any of the French operatives) can't possibly have been involved, as an armoured S-class Mercedes is so unlikely to be struck by a little fiat, especially as it is an enormous Mercedes saloon.
Do the maths and physics. A car travelling at even 40mph but in this case, up to 100mph, or maybe slightly less in entering the tunnel.
Tell me: what happens when a car driving at that velocity comes into contact with a solid object such as a tree or, in this case, a concrete pillar?
Hmmm?