I don't think I've ever heard anyone say London density is bad. London _sprawl_ is bad, and one problem is how low density London actually is. More density would mean more housing available near the centre.
Eh - I see diminishing returns around smaller public-ish transit (!) commutes. The difference between a 15 minute commute and a 45 minute commute wasn't significant for me. I can still pay bills, answer personal mail, read a book, and talk to friends on my way to work. I imagine folks at the end of the tube line in London feel the same way.
For a while my commute was 1.5 hours each way. I found it intolerable and moved, but I have coworkers who commute as long or longer. London is starting to sound good.