The 40% figure on the website quoted by Dangerman is a percentage of GDP, so includes everyone in the country be they employed, on benefits, seeking asylum, etc.
For someone who, like the majority of aircrew, is in the higher rate tax band, the last figure I heard quoted (sorry, can't remember when... few months ago, Radio 2, probably Jeremy Vine) is that on average, over 70% of your gross income eventually ends up in the treasury's coffers one way or the other.
Sounds a lot at first but it's quite plausible... consider how much you actually get to see in your wallet after your own income tax, NI, council tax etc has been taken out. Say, for example, you spent all that on beer (may even be a reasonable scenario). After the VAT and Excise Duty that goes straight to the treasury, some of the cost of each pint will go towards the salary of the lovely lass who pulls it, and some of that will then go to the government as her income tax, etc.
So one way of looking at it is that you only get to "keep" 30p of your hard-earned pound.
Dave