I wouldn’t worry about the 80% as it’s not really an HB matter. They will have applied their methodology, which might for example involve comparing the scheme with high-end mixed tenure retirement villages around the region, and in theory the regulator is content that their methodology is robust. It’s not our job to police the regulation of social housing.
However, HB does contain provisions to restrict unreasonable rents in various ways:
– If you consider the rent payable for the dwelling to be unreasonably high, you can (indeed must) get an RO determination and, if this is not exempt accommodation, HB will be limited to the LRR
– If this is exempt accommodation and you consider the rent to be unreasonably high compared with suitable alternative accommodation which is available, you can restrict the rent to whatever amount you consider reasonable. Good luck with that though – the evidence burden is almost impossible to satisfy.