An update:
Just picked up two 3 SIM cards. One doesn’t work.
The other one has given me a number and I’ve gone through registration. Confirmed number with 2fa. Trying to log in so I can buy a package and it then asks me to confirm my email. Have tried for last 30 mins with several valid emails on different domains and it won’t even let me submit.
Running out of time and will have to give up shortly…
Try the anonymous top-up
https://new.three.co.uk/account/anonymous-top-up?aemPath=anonymousTopUp&userType=Anonymous
You may just have to get a Free Mobile SIM once you arrive in France. They have an automated machine you can buy a SIM from in what appears to be a corner store at 94 Boulevard La Fayette in Calais (since you mentioned you’d be looking to pick something up there).
I’ve used that fine in the past but it’s not working from this SIM. I’m reluctant to top up via another device as it’s unclear how to buy a data package after doing so.
Am now going to Dieppe.
102 Grande Rue is the address in Dieppe if you want to do it at a machine, 22 avenue des Canadiens if you want to go to a store to sign up.
Enter the number choose payment method type and it lets you choose a bundle, select and then click make payment .
Lol it gets worse. Got the second SIM to finally let me top up using that anonymous link. It gives me these options:
So much for Three PAYG being cheaper. Weirdly the Three website at Pay Monthly SIM Only Deals – Unlimited Data & 5G Plans | Three offers 30 GB for £24 which is 3 times cheaper but not possible to buy with this SIM 
Is that a free 200mb SIM?
Looks like it’s on the mobile broadband plan so those are the add-ons available.
Must be. How do I switch it?
I don’t know of any way besides I assume customer service.
At the top of the page by the phone number does it say mobile broadband number 07xxxx?
If you can pickup a 3 SIM from a supermarket or corner shop.
If they are the free 200mb SIM then I think your registration issue is you have to register with the email address you ordered them on, so if you used a throwaway email that is probably the issue.
But if OP is leaving soon, then picking up a SIM in France is probably the most practical option at this point.
You want to buy a normal top up bundle not those as they look solely international stuff which is a con.
Provided it’s a payg sim not a mobile broadband sim, download the red Three app from the store I think, the white one is for contract.
Or just pop it in and call 333 or whatever the top up number is.
Back before the free roaming came in I used this site to search for foreign sim cards and found it useful and (mostly) up to date. France | Prepaid Data SIM Card Wiki | Fandom
Eu sims as well apply restrictions of how much data from your bundle you could use in roaming, especially unlimited deals. If your are going to travel to 1 country for an extended period of time you are usually better using a local sim from that country, but they are not usually easy to get before you arrive to the country.
