It's not closing down, from the article they are moving to newer furnace technology which requires less workers to run.
Both have announced plans to convert from blast furnaces to modern electric arc furnaces. This will greatly reduce emissions - they are among the largest industrial polluters in the UK (along with the Drax wood-burning power station). But conversion to arc furnaces also means that fewer workers will be required.
The UK exports 7-8 million tonnes of scrap steel every year, while producing about 5 million tonnes in blast furnaces. There's more than enough feedstock to replace all the UK blast furnace steel production with EAFs and still have some left over.