Dd has a doctor friend who’d moved to Oxford and let her London flat.
During a conversation with a married couple (both doctors in the same hospital) who also rented out at least one property, they told her that she was mad to be declaring her rental income. ‘We never have!’
IMO this is probably rife. If you don’t have a mortgage, don’t use a letting agent and don’t live in an area where registering with the council is mandatory, there is nobody you need to tell.
The self assessment tax form doesn’t even ask for addresses - it asks only how many properties you are letting.
And I dare say that at the lower end of the market, those slumlords who cram people in, will be demanding their rent in cash anyway.