One word of advice - don't. An eagle eyed Spanish or French cop could well spot the lack of tax disc and the moment they demand documents and cotton on to Spanish insurance on a British registered car with no mechanical certificate from anywhere, you're in very hot water. If you do get as far as the UK (your Spanish 3rd party cover will NOT now cover personal injury to you or any passenger, by the way) you can expect Customs to have a word about the tax disc and BIBs will most likely catch you on ANPR about the rest. I was once pulled over at Poole Harbour by HMC&E and asked all about my plans for my stay in the UK (I was returning to the UK for the summer) - my conclusion was that they'd checked the car's registration while I was on the ferry. I had three days' MOT and a month's tax to run at the time.

You won't get more than 100 yds from the ferry before being pulled over, and the book being thrown at you. AFAIK ANPR cameras are fitted at all ports - which would kind of make sense.

SORN means you've notified the DVLA that the vehicle will NOT be used on the road, hence it is no longer road legal in the UK. Operating the vehicle for temporary periods (i.e. on holiday) in another EU country is legal, providing the car is legal in its country of origin - which yours isn't.

And, yes, the Policia Local where you currently live probably don't care, but GC Trafico and the national police in France won't necessarily be looking the other way when you drive past! GCT will happily impound your vehicle leaving you to make your own way home...

Daft question... but why don't you get it MOTd in Dover?