It's called cannabis!

https://www.marijuanamoment.net/illinois-collects-52-million-in-marijuana-tax-revenue-in-first-six-months-of-legal-sales/

$52 million in sales and excise taxes  following legalisation of adult use on $239 million of sales in 6 months. Annualised that is $104 million in sales and excise taxes on $478 million of sales in the first year of a legal adult use market.

However many more adult use shops are opening up as the licences are granted and as more consumers switching from the black market to the legal market, so the annual figure is expected to be significantly higher (pun intended).

Population of Illinois is 12.63 million.

Population of UK is 66.6 million.

Assume 5 times the market in the UK would give $520 million in sales and excise taxes, or circa 410 million GBP just in sales and cannabis tax in the first twelve months.

Corporation tax and income tax from businesses and employees in the legal cannabis industry would be on top of this.

Plus police and court savings on decriminalisation.

Within a couple of years with legal market growth it could be a steady 1 billion a year in total tax revenue and costs savings to the government. Every year. 

You can build a lot of schools and employ a lot of nurses for 1 billion pounds  a year.