Disposable Income

The point were you say aha! I have a few quid to spare. Just when does an essential become a luxury? When do you feel poor? For some it will be not being able to send a child to private school. Others it will be only being able to rent a flat/house and not owning it, some saying bye to the car, holiday. What about only one pair of shoes, coat? Who decides the point of what is considered disposable income? What's a luxury item and not essential? The VAT office seem to know, but do they really know? Maybe there is no such thing as disposable income after all.