Reply from the Diamond Desk: "you earn 10 Base points per each $1.00 US Dollar paid, excluding taxes, therefore, the Base points earned are 1,158".
My spend excluding taxes (and tips) was 88.05 GBP. The pre-tax room rate was 74.90, and the total 107.00.
88.05 x 1.546 x 10 = 1158, apparently by fiat, because no attempt at calculation has been shown.