I managed to get Business Access by Marriott Bonvoy to show prices in GBP by setting my company's billing currency to GBP. However, I had to change the billing currency a few times until the setting seemed to take hold.

The reason I won't be booking through Business Access by Marriott Bonvoy is that, while the Business Access by Marriott Bonvoy rate is a modest discount on the Member Rate (about 1.5%), it's a much smaller discount than the Marriott Senior Discount rate (about 10%) to which I'm also entitled. Business Access by Marriott doesn't allow the Marriott Senior Discount rate to be selected.