If we somehow managed to stop all illegal activities and absolutely no money ever went into the black market, we would still be taxed at the same rate. The Gov would just find something else to spend it on.
Our taxes will never be lowered and we rarely get to say what its spent on.
I agree with this - things only ever seem to go one way. Even income tax itself was once a temporary measure, over 200 years ago, to pay for the Napoleonic Wars. In fact, have you noticed how there's never any question of whether we can afford wars or not, if the government sanctions them?
Look at things like VED, which used to be known as road tax - ostensibly to repair the roads. The roads are barely repaired anymore, but we still have an equivalent tax. Now it's based on emissions, but the government have made it clear that, once we're all forced to drive cars where the emissions are made in factories and power plants rather than on the roads as we drive, we will have to start paying per mile. Does anybody really believe that money raised by the ULEZ will go into genuinely easing pollution?
NI always used to be for pensions and healthcare, but that's gone by the wayside a long time ago. If you ran a different public organisation/charity company and it was discovered that your income was not being used as agreed, you'd be shut down; but when it's government, it's apparently all OK to make promises and agreements and then just fling them aside later.
Also notice how, whenever there are campaigns for new taxes or for people to not avoid tax, every single time, the amounts stated are based on how many more hospitals, nurses, teachers, children's centres, street lights we could pay for - and NEVER on potential increases to MPs' pay, their heavily-subsidised bars, their highly-questionable 'expenses', fomented wars that have nothing to do with us, excessive bureaucracy, vanity projects, supposed 'diversity' (actually divisive and extremely controversial) causes, even things like HS2. They know very well what the response will be, if they tell us they need more tax from us to spend on what they want and we don't. Like with the CFs who perpetually claim an inability to pay their own way in shared costs, but then turn up in brand new designer clothes, the magic money tree is absolutely there when the government wants it, but its existence is outright denied when the people want to access it.
It's the same strategy, but on a grand scale, as children asking for more pocket money so that they can buy more fruit, museum tickets and educational books; and the Prime, chocolate, Xbox games and phone credit that it will end up being spent on are also never cited.
Once a tax revenue is accepted as the norm, even if the supposed reason for it is long gone, the tax remains, unquestioned, in perpetuity. At best, you hear about a few tolls levied for the building of bridges being scrapped decades or centuries later, once the bridge is finally paid for; but the vast, vast majority of taxes are here to stay forever, regardless of any ongoing justification.
Inheritance tax has been mentioned, and that was initially supposed to break up huge family estates - one reason why the National Trust now has such an amazing portfolio. Now, we're in the position where any modest family home in London or the SE will breach it and mean that many ordinary families end up paying for it (I'm lucky enough to live in a perfectly nice but much cheaper region, before anybody accuses me of arguing with self-interest). Also stamp duty, which, if we must have it, if fairly levied, would manifestly be on the difference in house value when you trade up. Instead, you currently can move to an identical house in another town (or three streets away) - or even a significantly less nice/cheaper one, maybe forced to by COL and other family financial circumstances - and you're taxed on the value of the house you move into, with no allowance made for one you give up.
This is, of course, if you manage to avoid the obscene Elderly Tax, whereby the routine, anticipable health needs of a great many older folk have to be paid from the sale of their own houses. Nobody ever suggests that younger people who may have similar needs have a charge put on their/their parents' houses. People complain "Why should MY taxes have to pay for YOUR health needs, so that YOUR children can inherit your house?" - but this is precisely what the stated aims of the NHS are: cradle to grave healthcare, free at the point of use, provided according to need. Nobody would ever dream of trying to deny a similar potential later inheritance to, say, a child who survives leukaemia and thus incurs very expensive healthcare costs, before any family asset has been built up and is thus there right now for the grabbing; or even any suggestion of deferring the standard costs to the NHS of your own birth and many and varied routine childhood health/other costs to when you may be able to pay for them in middle age - because the public would never stand for this; but when it comes to the elderly (and/or disabled folk), far too few people care about them to protest against blatant discrimination against them.