Britain paid £270m a week, and received back £100m. Johnson invented a £350m figure, the entirety of which he promised to the health service, and then promised the £100m spending commitments would be met. So his new spending requirements are £450m a week, and the money saved £270m. In other words, he has pretended to magic £180m a week out of thin air.
He has also effectively promised major restrictions of immigration alongside access to the single market, which everyone knew was impossible, and promised there won't be a recession, which is unlikely. He gambled the country's stability for his personal advancement, and his duplicity and incompetence will probably lead to a far right party becoming a major force in British politics.
It is not surprising that people are angry.