You are right it is a tax , the government sees it as a very important way of getting more money out of wealthier people, they have the ability to reach into your bank account and grab your money too. 
. Thsy can track almost everything about you even easier than the revenue dept, especially after covid. It’s all linked together, who is related to who, your salaries , your entry exit date.
I had some issues where my whole family was put back on the higher rate for six months, my full rate, but wife has successfully put the kids back on her through the trade union. So now 3/4 of us are a much lower rate.
These are unions like hairdressers union or fruit vendors union or whatever, and they are legal. You have to pay your subs through them every month. Some of them are a bit dodgy in my opinion, for instance they only accept cash transfer, but it is accepted by NHI.