American Express in the UK are emailing members who collect Membership Rewards telling them how to connect their Amazon Account to allow them to use their MR points on Amazon purchases. You can redeem points in full or part payment for goods normally available on Amazon.co.uk .
When linking accounts you need to have a UK Amazon account with an address which matches your Amex billing address. Of course you can create a new one in your Amazon account if needed.
Once you have matched the screen explains how you can redeem points during the check out process.
I am not sure that 2 points for 1p of saving is a terribly good deal, but if you have some left over points this would be a good way to cash them in.