You have to sign up for Amazon Advantage UK, but don't go any further than creating the account. (Don't enter any financials, for example.) Remember, you're not actually going to be listing and selling products, just advertising with AMS, so you don't need this to be fully configured.
You have to use a different email address to your KDP/Amazon UK signin.
After that you can join AMS using via Amazon Advantage UK, and *as far as I know* you don't have to pay anything. (They haven't charged me a monthly fee, but then I didn't put in any financials. Maybe at the end of the year they'll go 'ello, 'ello, 'ello wots all this then?)