Mark,
I have some experience of your situation from when I was living & working in Paris a couple of years ago.
In terms of registering in France, and from my own experience of trying to register my wifes RHD Renault Clio, you first need to go to your local Prefecture to obtain the appropriate application document, but the key document they need is the vehicles Certificate of Conformity (Certificat de Confirmite) which you 'should' be able to get from Porsche GB. This is the essential document that the Prefecture needs to issue you with a registration number & plate (plaque d'immatriculation) and Carte Grise (their equivalent of the V5). My Prefecture didn't mention any need to change over the lights or the speedo etc, providing we could produce the CoC document....hhmmm!!!
However, in parallel, I believe you also need to advise the DVLA that your car has been exported abroad, which means that the vehicle is then deleted from the DVLA records. In the end, I couldn't be bothered with the French registration/DVLA process as we were only going to be there for a couple of years so I brought the car back to the UK and in the meantime bought a Twingo for my wife to run-around in.
But you are right about the 90 days insurance cover limitation. Have you asked your insurance company in the UK if they offer an Annual Green Card which provides unlimited use in Europe (at least with my insurace company)? Not sure if it is intended to be used in this way, but if you are only there for a short time this could we be a way round the problem, although you would still need to use a UK address and ensure the car is taxed and MOT'd.
However, I am still in Paris regularly, and in the suburb where many ex-pats tend to live I often seen english cars bearing their original UK plates but also showing the little green French insurance vignette in the windscreen. So it could be that things have changed/relaxed in the last few years - also worth looking into, perhaps?
You also asked about insurance companies. I used an adviser (like a broker in UK) for all my insurances, and found him to very competitive and extremely efficient. He deals with all the main French insurance companies - our car was with Axa Courtage, but we also had quotes from the likes of MACIF etc. Here are the details in case you would like to follow-up :-
Alain Combelle (Assureur Conseil)
31, rue Houdan
92330 SCEAUX, Paris
Tel : 01 46 60 59 27
In case you don't speak French, ask to speak to Madame Combelle whose English is fluent.
If I can be of any further help do let me know. Good luck.
Jeff.
