Treasury is the name for the government's economic ministry, ie the people who "have the money" (except that of course all modern governments run a huge debt) if you're American your equivalent department is also named Treasury.

The UK's tax authority is HMRC, His Majesty's Revenue & Customs.

It takes a lot of time to build electrical generators, so "rapidly" here means what... decades? Years at least. I think wind farms generally you need to do a bunch of paperwork and then if you get an OK (after the paperwork) maybe 3-5 years to build.

The weirdness you're talking about is the other side of the Contracts for Difference subsidising the off shore (and historically onshore too) wind farms. A CfD works like this: You auction off the right to build generation and in the auction people can bid down for the price they'll be paid for say, 10 years of their electricity, this is called the Strike Price. Whatever they sell their electricity for, they always get that strike price. When the sale price was lower, the government is giving you free money - that's why this is obviously a subsidy. But when the sale price was higher the government (effectively the treasury, though actually via a for-purpose government owned company) takes every penny above your strike price, too bad.

This subsidy is cheap for governments because it's about certainty, something they have and which private investors lack. The British government knows it will have tax revenue in 2036, but a private investor would want a fat premium to cover that.

Now, CfDs run out. If you have 10 years of CfD obviously the wind turbine you bought doesn't magically explode after exactly 10 years, maybe maintenance prices get out of hand or the main blades reach end of life in 20 years and so it doesn't last forever, but eventually there's an unsubsidised generator, the situation today though is that there's a lot of very new generation, and so most of it is subsidised.

Another issue is that it makes sense to build off-shore wind farms in particular on the Scottish coast, whereas it didn't make sense to build e.g. coal generation there, so the UK isn't set up to move a huge amount of power made in Scotland to the south where much of it is needed. This results in a situation where there's say 15GW of almost free electricity, but 5GW of it is the far side of a 2GW transit point, you can only have 12GW of that electricity, even though you made 15GW. Fixing this will take years and political will.