Biosafety level 4 (BSL-4) is the highest level of biosafety
precautions, and is appropriate for work with agents that could easily
be aerosol-transmitted within the laboratory and cause severe to fatal
disease in humans for which there are no available vaccines or
treatments.

Dr Trump, the US's self-proclaimed expert on medicine, and inventor of
two possible cures for Covid-19, tells us that he has seen evidence
that the virus emanated from the Wuhan Institute of Virology, one of
two BSL-4 facilities in China. Others, such as the US intelligence
services, say this is just an untrue conspiracy theory, but there
remains the possibility that the virus was under research in, and
escaped from the Wuhan BSL-4 lab.

The lab was built with French assistance, but they fell out over the
planned cooperation, and so the lab was not certified by the French
experts, who also were not seconded there as planned. The French
partner is in Lyon, and is one of three BSL-4 labs in France.

This got me wondering how many such labs there were around the world,
and how many we had in the UK. I discovered that quite a few countries
have them, but only three countries have more than three such labs:
obviously the US has the most, with 13, but the UK is second with
nine, and Germany third with four. Australia has three, all in
Victoria, but Brazil, Canada, Italy, Russia South Korea, South Africa,
Sweden and Taiwan only have one each.

So where are the UK labs? I guessed that Porton Down would be
prominent, and it does indeed have two such labs, one military, one
civilian. More surprisingly, there are three conveniently located
BSL-4 labs along the Northern Line, and three more in Surrey. There's
also one just north of London, in Potters Bar. So London and the
surrounding area has more BSL-4 labs than any country in the world bar
the US! And the Northern Line alone has more than most countries.

https://en.wikipedia.org/wiki/Biosafety_level