Right now, there's lots of debate about this tiny 1% of people who may have to pay tax and the view seems to be, "screw 'em. They earn squillions so they can bloody well pay". But as lots of people have pointed out, how much more does this REALLY raise when you consider that it is only 1% of the population?
So then, let's extrapolate that a few months/years after they impose this tax, then they'll say "Mmm, okay, that worked out. 1% of the population was a bit cheesed off and everyone else was happy. let's see if we can now increase the actual amount we get by saying everyone who earns over 100k must now pay 45% tax".
You see where I'm going with this? This tax is easy for government because it makes the 99% of the population who don't earn that happy that those rich feckers are being punished but... it doesn't actually raise more cash. So you just wait - labour wins the election off the back of everyone's happiness that the rich are being screwed and then see what happens....