On forms that require your UK postcode, try ZZ99 3VZ – it’s the legal, valid postcode for ‘No fixed abode’, which typically means you’re homeless.
Forms need two things:-
‘What is your name:-‘
‘What is your address:-‘
People who know their postcode or zip code, if they have one, will put it in the right place, otherwise, they won’t.
Try living somewhere without a county, too.