Hi, so the car is still registered in the UK? (but SORNed?)
I am not really sure there is a time limit to register a car in France, as long as you are the one importing it from abroad and you have an invoice of some sort. What you really don't want to do, is buying an unregistered car from a guy who brought it to France and didn't bother or failed to register it. You have to be the original buyer who bought it abroad. You then have a few weeks (or maybe months, can't remember) to drive it with a import register plate from the original country. Maybe a temporary registering is possible in France too, not too sure about this (WW plates)
When I bought the Esprit from Belgium (it turned out it was an original French car so I brought it back \o/) I registered it way later and it was not an issue at all (but I am talking about months, not 10+ years)
If it was already yours and you just brought it with you, I am not too sure what you should have done, probably a simple procedure.
I can have a look at a *.gouv.fr website if you are not too comfortable that stuff in French.
My guess is, as long as you have all the regular papers to register it, it should not be an issue.
Then again, you may register it as a classic car as it's older than 30 years old, and it's easier. For an example, some people succeed in registering a barn find without the original papers at all, so .......
Now, I don't know if that would be a good idea to register it as a classic to bring it back to the UK: maybe it could be more complicated? Although I think it should be doable too.
edit: could you just not un sorn it and get it back?
Is a sorn (ed?) car supposed to be parked in the UK? When you unsorn it, do you have to prove it's here in the UK with you?
edit2:
https://www.rac.co.uk/drive/advice/lega ... sorn-mean/it doesn't say anything about a car being SORN abroad? (whether you drove it or not is not really relevant as they can't know if you did anyway).
So couldn't you just un SORN it and put it on a trailer?
I may be missing some details from the SORN status ^^'