It's jealousy pure and simple.
Regarding tax avoidance, my take on it is that perceived unfairness of the tax system. Take income tax for example:
Everyone gets to earn £8k tax free (although I think this is changing soon), then from 8K to 35k is taxed at 20%, 35k to 150k is taxed at 40% and top rate of over 150k is taxed at 50%. (Then you get taxed on your SAVINGS interest - that's money you've ALREADY paid tax on when you earned it by the way.)
Take someone earning 30k, they will be paying about 5,200 per year in income tax. So surely someone earning 60k a year should have to pay 10,400 per year income tax right? I mean, that's fair isn't it? You earn double, you should pay double.
So will somebody please tell my why the hell that person earning 70k should have to pay nearly £18,000 in income tax? Why is that fair?
The person earning £70k would still be contributing double the amount of money to the treasury and if you think about it, is probably taking less out as well. People with more money are less likely to use subsidised public transport, are more likely to have private healthcare, more likely to use fee paying schools and so on and so forth.
When you look at it this way and then go and watch some of the tv shows like 'benefits street' where you see money being thrown at layabouts left right and centre, it's easy to see why the 'rich' decide they want to employ some clever accountants and reduce their tax bills.
It's really unfashionable to be seen as 'rich' in this country, as if being successful is something to be ashamed of and punished for..