Thank you. All that will happen to these items is that they will eventually be scrapped and therefore not letting you have the Jaguar parts obviously makes no sense.
If you have been paid out for the theft of the car on your insurance, regardless of the identity of the parts, they do not belong to you but they belong to the insurance company. You do not mention that but I thought it was worthwhile mentioning the point.
It may be that you have bought the salvage of the insurance company in which case the part belong to you.
The fact that you say there was only one Jaguar recovered makes it seem unlikely that any of the parts belonged to another car in another location..
The cost of litigating with the police (who do seem to be blinkered with issues like this) is probably going to be prohibitive and therefore rather than rush into expensive litigation the first thing I would try would be a letter of complaint to the Chief Constable and if that fails, a complaint to the Independent Police Complaints Commission.
The common law situation is that these items (if you can prove on the balance of probabilities, more than 51%) that they are yours, the civil court would award them to you. Whilst the theft is a criminal matter, ownership in this case is a civil matter.
However even though in these circumstances it seems obvious that these parts have come from your car, if the police steadfastly refused to release them to you (and the insurance company has no interest in them) you are faced with either complaining or litigating.
Can I clarify anything for you?