Any question answered...

Maybe during slum clearances? Totally guessing with no idea, but there might have been some compulsory purchasing in return for new housing in council blocks?

Good question; and how did they all go?

Complete guess but bomb damaged areas might might have been bought up.

Slum landlords being kicked / bought out another possibility.

Westminster manages a bunch of them near us.

How did local government / the council come to own Victorian terraces in London?

A couple of reasons--look at 20th century housing estates and their footprint. Many more were envisaged and councils bought up small sites with a view to demolishing them and combining the land into larger new-build estates. Then the tide turned against councils and in favour of reconditioned older housing.

Another reason was that huge swathes of houses were uninhabited in London in the past couple of decades (the population fell continuously from the time of evacuations during the war until the early 1980s), so that rents were low, and private landlords often neglected their properties completely instead of going to the trouble of repairing and letting them. It was often necessary for councils to acquire houses and do basic maintenance so they wouldn't fall down, or to secure them so that they were harder to break into, or to turn them into council housing. With Thatcher's assault on local authorities, the latter became largely impossible, but some councils kept hold of housing in hopes of better times. Hackney still did major sell-offs in the early 00s.