Well, most of our salaries go into VATable products. Average household income of non-scrooging, mortgage paying family in UK is £52,400 per annum. Over 60% of which is spent outside mortgage and savings. Which means that unless we presume majority use black market and grey import for their weekly shopping, most of us in UK will save between 700-800 pounds a year on 2.5% lower VAT as a rough estimation. Thanks for little mercies, maybe, but I'm sure as hell not going to leave that kind of money behind on a counter.
Your maths is out. Even if the entire 60% was spent on VAT rated products it would equate to £670. This is of course if you didn't spend any of your outgoings on:
Petrol (will cost slightly more now as the VAT cut doesn't cover the duty increase).
Gas and Electricity (Not charged at 17.5%)
Non Luxury food (which is most staples)
Alcohol (Duty increase, again probably more than the VAT cut.)
Childrens Clothes (VAT free)
So it isn't going to help even the reasonably OK family on £52k a year. It will do bugger all for those on much less.
It is a con, a feelgood measure that will literally do nothing for families. It may help some small business but only if they don't pass on the cut and therefore get lambasted for "keeping our money" instead.
Unfortunately it seems that people are actually falling for the spin and thinking it is a really good idea.