Yeah the NHS BMI calculator isn't great, however most people with a large mass of muscles tend to already be aware that they are heavier due to having large muscle mass.
BMI calculators are generally used by folk that have average muscle mass.
I guess it all depends on where the fat (if it is fat) is stored as well. Each to ones own, I guess.
I believe that most folk that use a BMI calculator tend to have average muscle mass, as I have absolutely no evidence/source to prove or disprove that.