Even changed my default location to US, when trying to pay with PayPal I get:
Use a different way to pay
Your purchase can't be completed with this payment option. Try a different or new way to pay."
I've created a fresh US account, linked it to my UK PayPal on my 360 and the payment option refuses to show up no matter what on Xbox.com, making it impossible to "buy" the trial.
Looks like it's not gonna work for UK, if anyone else gets anything different let me know.