What do you mean by "primary coverage"? If Sixt included CDW in the price you would not have to pay anything, nor involve the card company.
That’s probably a good question - I just assume the UK requires liability coverage in the base price. I was advised by all 3 credit card companies to simply decline the optional CDW coverage, book and charge the car on their card then relax; they will cover any damages up to 50k. As I said all 3 seem very similar- I’m thinking AX might be bit better? I only bring this up because we just get back from NZ and I did scrape a guard rail, in that instance the optional CDW was very reasonable so I took it. They were great about the damage, I filled out a form explaining what happened took about an extra half hour.