It's been many years since I was anything resembling a tax accountant, but there are a few things I've never forgotten, and the distinction between tax evasion and tax avoidance is one of them. Evasion means doing illegal things to pay less tax (not declaring income, lying about expenses, etc), and avoidance is anything that isn't illegal (including taking advantage of loopholes). Evasion is wrong and you go to jail. Avoidance is both legal and acceptable, no matter what oh-dear-my-party-no-longer-loves-me Nick Clegg thinks.
The moral of the story is quite simple: the government makes tax laws, and if there loopholes, it's their own damned fault. They can fix them, and until they do, they should stop moaning.
I googled up a quote or two I remembered from all those years ago, judgements from British and American court cases:
No man in this country is under the smallest obligation, moral or other, so to arrange his legal relations to his business or to his property as to enable the Inland Revenue to put the largest possible shovel into his stores. The Inland Revenue is not slow - and quite rightly - to take every advantage which is open to it under the taxing statutes for the purpose of depleting the taxpayer's pocket. And the taxpayer is, in like manner, entitled to be astute to prevent, so far as he honestly can, the depletion of his means by the Inland Revenue.
Anyone may arrange his affairs so that his taxes shall be as low as possible; he is not bound to choose that pattern which best pays the treasury. There is not even a patriotic duty to increase one's taxes. Over and over again the Courts have said that there is nothing sinister in so arranging affairs as to keep taxes as low as possible. Everyone does it, rich and poor alike and all do right, for nobody owes any public duty to pay more than the law demands.
Nicked from this page; citations omitted because including them would probably be a bit pretentious.
(PS. The term 'avoision' was coined by some dumbass apartheid-era South African government minister. The desire to blur the lines burns strong in politicians faced with lots of voters to please and not enough money to do it.)