Looking back over the experience, I've learnt a couple of things about moving cars from France to England.
First thing is don't cancel your French insurance. You might need it to prove that you have insurance to the police if you get stopped. Whatever you do don't have an accident, your French insurer probably won't pay out.
Don't underestimate the time it will take to get your headlamps changed, get a certificate of conformity (if the car is French) and an MOT. You need all these before you can register the car. Then, look for insurance because the insurance company will probably only give you a couple of weeks before you have to register the car and provide them with a UK registration number.
In the end I used Adrian Flux (insurance broker) who not only gave me a good quote and cover over the phone but also gave me full NCB and were happy to provide insurance using the VIN number, provided that I registered the car quickly.
Following Craig's advice I went to our nearest DVLA office and was able to iron out a few small items which would have been much more difficult by post. Result.
And, hot off the presses, Fiat France, the new Chrysler, the new Jeep, returned my cheque for the certificate of conformity because evidently they only charge if you are importing a car and not if it's being exported. Double result.