Apart from the Sunday roast you mentioned, which incidently is available in the majority of pubs I've been to (I had a very nice one yesterday) what other parts of the "english cultural" experience are you missing?
It also used to be that you would have chip shops and sandwich shops and cafes that would serve an english breakfast. Now you have chain espresso shops and ramen shops and sushi shops, etc (hey, I live in Japan and love Japanese culture, so I will stick to complaining about that so as not to appear racist ;-) ). When I'm back in London these days I have to go out of my way to find the kind of food that was around every corner 20 years ago.
As for pubs, chains like Weatherspoons (love them or hate them) have done a great job of maintaining and even improving certain things like the availability of real ale. They have worked hard to lobby for reduced prices for beer so that it is still (barely) possible to drink a decent pint and not go broke. But the good independent pub in London is a rare sight now, IMHO. 20 years ago the words "free house" actually meant something. Now you'll see "free house" on the side of Champions League football entertainment centre that serves nothing but InBev products.
These days the city never sleeps. It never has a day off. You can obtain any kind of food or experience you like, practically 24/7. You can easily find high quality European restaurants. You can drink fine wine in the shop on the corner. You can find italian fashions (and pay your entire salary on a single outfit). These are all great.
But it's hard to find a quiet neighbourhood pub. Piano singalong? Ha ha ha ha! Wait less than 30 minutes for a good english breakfast? Drink a finely crafted English pint with Fuggles and Goldings hops? Find a non-chain pub that serves bangers and mash? Eat a pickle and cheese sandwich (even on a train)? Be able to not be disturbed at all hours by rowdy drunks when you live near a pub? All of these things are getting harder/impossible to do because they have been supplanted by non-English culture. Which is fine in one way, but like I said, I miss London of 20 years ago.