I know there's a regulation that doesn't allow VISITORS to apply for FLR status, but no idea where BUNAC stands in this. In my opinion, BUNAC is just a fancy visitor's visa, since it's still under 6 months, can't be extended, and has restrictions. You can enter the UK on a visitor's visa/VWP (or BUNAC?), then leave the country to apply for your FLR and then enter as a spouse. Someone else (probably Victoria) will have to clarify this.