Examples, please??
And what are you classifying as "slum"?
Maybe you're thinking of Victorian terraced housing, the stock that was built quickly during the 19th century in cities due to the industrial boom? They were certainly durable, but to last "hundreds of years"? No. You would be mistaken. Many of them are still around, though, so they're doing well, but you're talking about being deliberately built to last that long. Are you going to offer evidence to support your assertion?
Maybe you're thinking that post-war housing was designed to last "hundreds of years"? If so, you're mistaken. Maybe 50-100 years, but they were basically built to solve an urgent housing need due to many cities having the proverbial bombed out of them during that little kerfuffle with the Germans.
Maybe you're thinking about post-1950s housing stock? If so, again you're mistaken, as they were built for may 30-80 years. If renovated they may last longer, but they were not "built to last hundreds of years".
Now, there's certainly some buildings in the UK built to last centuries, but they tend to be the grand buildings. Stately homes, monuments, abbeys, churches, etc.
And, sure, there are run-down areas in the UK. Slums, if you will. At one point there was a street in Liverpool (iirc) where you could buy a house for next to nothing. Some estate agents were offering a "buy 2 get 1 free", because the area was so run-down. But the buildings themselves were still there (minus the glass in the windows). They would certainly have needed renovating to bring them up to modern standards, redecorating etc, but the issue wasn't, and rarely is, the housing/building stock itself, but the wider economic situation of the area. Deprivation, lack of investment in infrastructure, lack of employment etc.
So what, exactly, are you referring to? An example would certainly help clarify.