They (ONS) define disposable income as “the amount of money households have available for spending and saving after direct taxes have been accounted for. It includes earnings from employment, private pensions and investments, as well as cash benefits provided by the state”, so not exactly post-tax income, but £36.7k corresponds to about £48k gross. I completely agree that a couple earning min. wage should not be classed as high income in the UK, but I usually overestimate how much income a typical Brit actually has.