This is an utterly baffling case with so much unexplained behaviour by the victim and so many incredibly confusing clues.
The car ending back up in West London in the area of the victim's home/work feels very significant. If she picked up a north-bound hitchhiker and the hitchhiker attacked her, it would presumably have ended up somewhere north of the attack. And if someone pulled into the layby while she was fixing the tire, they would have had their own car and both the car and the victim would have been left.
The car was taken from the crime scene, almost certainly, because the attacker needed it. You don't risk getting caught driving a murder victim's car unless you have no choice. And it ended up in West London because they needed to get back to West London, where the journey started. And this doesn't speak to a random attack.
The whole tire situation is also bizarre. I'd love to know the circumstances of the original tire puncture (the one that was repaired/picked up on the day of the murder) as it seems very strange/unlikely that the same car had two unconnected flat tires in the span of only a couple days. Was there a previous car sabotage/murder attempt that didn't quite go off as intended for some reason?
Certainly it seems like the police thought the husband was responsible based on their comments at the time and based on the fact that he was held for 60 hours at one point, but just couldn't pin a case on him. And his movements are unaccounted for in France for a big chunk of time. You wonder if he could have made the 90 minute flight from Paris to London on a fake passport, 'surprise I'm home!' and told the victim there was an emergency at the Cambridge property and they needed to meet the developers Saturday morning, both get in the car, he commits the murder, drives back, hops back on a plane and nobody is the wiser.
But this seems a bit farfetched (more mystery novel plot than real life), it doesn't explain the weird number plate thing that I'm not even going to try and guess at, and the awkward car jack being used as the murder weapon does suggest an unplanned attack in contrast to almost all the other evidence. Ugh.
The number plate thing might be the single strangest clue I've ever come across in a true crime case. It seems totally legit - apparently he went into one shop, was referred to the shop next door and both accounts jive - but I cannot think of any possible explanation for why anyone would want extra plates for a murder victim's car after the fact. And I don't believe that the murder victim had even been identified at that point, nor was any significance to the car realized. So weird.