I've often wondered, and yesterday heard this from daughter. She has a friend who's a doctor, quite senior, and has some hefty expenses looming. She also has a London flat she has rented out since moving to the area.
She was discussing finances with another doctor, who asked whether she paid tax on her rental income. She said yes, of course, she couldn't sleep at night otherwise.
Other doctor's reply was basically, 'Then you're mad. We never have.'
This must be so common - I do wonder why on earth no govt. has cracked down on it.
Who on earth is to know if you don't? AFAIK there is no system at all for registering who rents what out, and the SA tax form asks only how many properties and what the rent is - it doesn't even ask for addresses. And who's to know even if you declared rent with a good percentage knocked off?