Yes, you might be right but the fact is at the moment raising tax does not necessarily increase tax revenue for all reasons mentioned. It is simply not as simple as left assume. Putting your measures in place, raising taxes, lowering threshold would make the government of day even less popular...and could quite easily slow the economy, especially if Labour did stick to leaving EU...they,d be out within a couple of years.
The real solution is to foster growth and increased tax through greater earnings then we attract foreign investment and retain high earners. You can not operate a non capitalist successful nation in a capitalist world .
Tax avoidance is only possible because it is allowed....I note it is only the very wealthy who can do this...it is something confined to the rich. If any Government was serious they could eliminate this by making it illegal......it would make Lord Ashcroft very annoyed of course.
Raising taxes need not make the Government unpopular as the example of the Scandinavian countries show. People there appreciate the social support structure made possible by increased taxes.
You seemed to be equating raising taxes with the demise of capitalism....heaven knows why. It is quite possible to raise taxes in a capitalist country.
"The highest rate of income tax peaked in the Second World War at 99.25%. It was then slightly reduced and was around 90% through the 1950s and 60s.
In 1971 the top rate of income tax on earned income was cut to 75%. A surcharge of 15% kept the top rate on investment income at 90%. In 1974 the cut was partly reversed and the top rate on earned income was raised to 83%. With the investment income surcharge this raised the top rate on investment income to 98%, the highest permanent rate since the war. This applied to incomes over £20,000 (£191,279 as of 2016),
[7]. In 1974 750,000 people were liable to pay the top-rate of income tax.
[18]"
https://en.wikipedia.org/wiki/History_of_taxation_in_the_United_Kingdom
Compare this to the situation today.