As someone who deals with ANTS on a daily basis I cannot understand why your UK licence is not accepted.
As a matter of interest, when you say you sent “all the required docs”, is this because the seller did not give you a code de cession? It is a ten character code which you just enter into the box provided when you click on “Acheter ou recevoir un vehicule” on your Demarrer une nouvelle demande" page.
If you don’t have this code you should still be able to register your purchase easily without uploading any documents at all.
A driving licence & insurance certificate are only asked for in about one in ten cases, certainly not every time.
You are not required to pay anything until the application has been completed by ANTS but agencies, some of which give the appearance of being the government site, usually demand full payment of the registration tax before commencing.
How do you sign in to ANTS? Do you have a user name & password or do you use FranceConnect & enter your tax or social security number?
The only reason ANTS would refuse your UK licence would be because it is a new one issued after Brexit.
With regard to your imported car, have you declared it to Douanes & have your 846a certificate? If it is over 4 years old you will also need a CT which needs to be less than 6 months old. You will also need a bill of sale (which must have the VIN of the car on it) & correctly complete the cerfa13750 demande form. Make sure the uploads meet ANTS’ file type & size requirements & don’t use Google Chrome as some versions are not compatible with ANTS. And don’t submit your driving licence, insurance or certificate of conformity unless ANTS message you later & ask for them.
Below is an example of the sort of message ANTS would send you (in this case it is a Spanish import & the importers did the wrong thing & left the registration document behind). ANTS ask for a driving licence but do not specify that it needs to be French (they never do).
