I've found a rate on a third party site for a Marriott property for £192 per night. B&B, non-refundable.

Marriott is offering the same room for ~£254 per night, B&B but is the "Member Flexible Rate with Breakfast".
If I choose the "Prepay and save" tab, the rate actually goes up to ~£296 per night, for the same room and B&B, nonrefundable and nonchangeable.

Would this final rate be eligible for the BRG? Seems too good to be true.