I've been living there for a while now and as much as there good things about London, being clean is absolutly not one of those... Unless you stay in the really wealthy areas. Where I live the really few public trash are never emptied, their content being scattered by foxes every night and people tends to discard their take away boxes on the pavements and roads where it stays for quite a while.
The stones being dirty is another issue. A lot of white stone will get darken by car pollution really quickly and cleaning those is a never ending process until fossil fuel is completely banned. The speed at which the the stone will get dirty as the the price of cleaning it wihtout damaging it differs a lot depending on the type of stone. I know that around the Loire valley, cleaning tuffeau is a huge pain.
I was about to write the same thing. I lived in Paris and now in London. Often residential streets are disgusting and smelly because people just leave their trash bags there out in the street.
Talking about stones, you can easily see the dark layer of pollution on bricks as well.
Talking about the Loire valley, I lived in Tours also and It's true that this is very difficult to clean, if I'm correct the best technique is to use a laser but it's slow and expensive.
The stones being dirty is another issue. A lot of white stone will get darken by car pollution really quickly and cleaning those is a never ending process until fossil fuel is completely banned. The speed at which the the stone will get dirty as the the price of cleaning it wihtout damaging it differs a lot depending on the type of stone. I know that around the Loire valley, cleaning tuffeau is a huge pain.