New Visas are being introduced for skilled/semi skilled: https://aristonesolicitors.co.uk/news/new-uk-visas-set-to-la...
UK is NOT trying to emulate North Korea. This is a bad faith statement. Those countries (USA, China, Singapore) you mentioned are not in the EU Common Travel Area either.
On trade, the UK is negotiating new trade deals: https://commonslibrary.parliament.uk/research-briefings/cbp-... A fair criticism is that this hasn't gone far enough yet.
I think Indians might question calling what the East India Company did to India 'trade'.
"stopping skilled/semi-skilled immigration. This is completely and astonishingly backwards."
britain was an emigration nation for almost 500 years, immigration had nothing to do with Britain acquiring wealth, you have this part backwards.
1. Bring industry in and let skilled immigrants come in so as to create a modern high skilled economy.
2. Emigrate out en mass to other countries that have successfully created modern high skilled economies.
Most countries would die to have number 1 in their own shores.
For the vast majority of British history, Brits were only wealthy when they traded outside. The trade took various forms such as East India company, trading outposts, industrial trade outside the island, colonization, financial capitalization etc. Britain ruled the seas (and still does behind America), and established English as the lingua franca of world business.
Yet, people keep voting for inwardness. Brexit, tax cuts for the rich, stopping skilled/semi-skilled immigration. This is completely and astonishingly backwards.
The only way Britain survives the competitive world is by trading more and making itself a hub of education, engineering, finance and global businesses. Constantly voting for restrictive trade, restrictive borders and disconnect from the rest of the world takes Britain closer to North Korea than to USA, China, Singapore, Australia, Canada - who are all trying to forge more relationships with the world.