Hi all,
First question: If one wanted to, is it possible to keep one’s UK- registered car in France for longer than 90 days?
Second question: If one was to keep their car in the UK longer than 90 without the appropriate permission (assuming there is such a thing), what would the penalty be?
M.
In reality (whether strictly legal or not) if you’re just on holiday (90/180) probably nothing to worry about. The only problem you’re going to have is with insurance.
Yes, and insurance is required even if you never take it out on the road, unless you disable it somehow, apparently. 
If the UK reg car is owned by a resident of FR it’s illegal to drive it in FR [unless you have a confirmation of application to reg to FR plates with you in the car] or UK.
UK reg car in FR driven by FR res and FR res driving in UK reg in UK are the problems. Number of days doesn’t come ito it.
I don’t understand the 2nd question. A UK reg car, with MoT and insurance, is legal in UK if the driver is a UK resident. If reg resident elsewhere, the driver is committing an offence, as I understand it.
That’s why my Danish pal had me along to drive when we got to DK in a UK reg van. I was res UK at the time. To be legal when we got to DK if he was driving it he would immediately have to declare it to Customs, pay the import dues and apply to reg it to DK plates. As it was, it was coming back to UK.
Assuming that you are talking about a non UK registered vehicle being present in the UK, then the limit is 6 months, which can be one continuous period, or the sum of multiple periods within a span of 12 months. If this period is exceeded the vehicle is deemed to have been permanently imported resulting in import duties becoming payable. Failing to comply could result in substantial penalties plus seizure of the vehicle.
By the way. A UK vehicle registration document does not give details of the vehicle owner, but rather states the name and address of the habitual keeper of the vehicle. The vehicle owner could live anywhere (including abroad), or indeed be a lease company, but the person who normally keeps the vehicle at a UK address is the one with their details on the registration document.