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.
I am afraid this claim is most likely to be rejected. As per the terms and conditions "When multiple rates for the same hotel, reservation dates, and room type are available through a Marriott channel you must reserve the lowest rate for that room type to be eligible for the guarantee.". Since the 254 GBP is the lowest rate for that room type that includes breakfast, you would need to book that rate. As this rate is flexible/refundable vs. the non-refundable rate on the competing website, your claim would be rejected.