From what I have been hearing a La Poste varification (not a La Poste bank account) does not seem sufficient proof for residency & will often produce “erreur 12”.
Another problem - you cannot mix n’ match UK & french regs as you have now discovered! You french insurance agent was right, french insurance does give you automatic europe wide cover without the need to notify the company unlike UK cover. But it would require the car to be fully french compliant in all other aspects. (You cannot take your french car abroad on a temporary carte grise, for instance).
For some years now you have been obliged to register your car in the country where you live, so you should have registered your car here. If you brought it over with you when you moved you would be allowed 6 months do get this done. If you were already a french resident when you brought it over you have one month. This really means that you have one month to start the process. French insurance companies, in common with other EU members, cannot provide insurance cover for more than a month although most will extend the cover at their discretion if you are in the process.
Generali have this on their (out-of-date) web site " Here in France we are able to offer:
cover for foreign-registered cars for up to three months. This initial period can be extended by a further 3 months on the condition that the request to import the car has been made at your local prefecture during the initial 3 month period."
No french company can offer a longer cover.
A UK reg car, to be legal for use in Europe, must be UK taxed, UK MOT’d, insured by a member of the MIB (UK Motor Insurance Bureau) & owned by a UK resident.
Your difficulty with the UK authorities stems from the Road Traffic Act 1988 well , this part anyway -
(2)The policy must be issued by an authorised insurer. And
(5)In this Part of this Act “authorised insurer” means a person or body of persons carrying on insurance business within Group 2 in Part II of Schedule 2 to the [1982 c. 50.] Insurance Companies Act 1982 and being a member of the Motor Insurers’ Bureau (a company limited by guarantee and incorporated under the [1929 c. 23.] Companies Act 1929 on 14th June 1946).
No french company is such a member. Companies might have the same name such as Axa but they are different companies in the named group.
Lastly, if you are at your wit’s end with the ANTS site you can always ask a friend to do this via his access or pay an authorised garage to do it for you. Not all such garages will do imports though so you might have to shop around - just don’t use an online company!