Found this morning on my Amex Plat Cashback, also on supplementary card and Amex Gold Reward card.
List of participating outlets: https://amexoffers.uk/starbucks/
Up to £5 cashback in total.
Should be able to stack with any in store offers (e.g. buy £20 gift card get free drink at Starbucks McArthur Glen Mansfield).
Works as an app top up (thanks to parky200)!