Of course the movies that Anna starred in were an "exaggeration" because those were not the point. They were the ones that got her to where she was - a very famous actress whom everyone wanted to be with.

I don't think anything in this movie is meant to be realistic. Even the movies that Anna Scott starred in were an exaggeration.

it is true that house would have been vey expensive even in 1999. however, a few years earlier, in the mid 90s, house prices in the UK went down very low, even in London. ir is not impossible that he and his then wife could ahve bought it if, say, she had a very good job, and he perhaps had some money of his own, from an inheritance say. if it was 2.5 million in 2009, it could have been about say 500,000 in the mid 90s.

I just watched the film in its entirety for the first time. I've only seen bits and pieces but I thought all those things were very obvious.

I hadn't seen the movie in years, but I LOVED that they moved into the little neighborhood with the private park and that's why they were able to hang out and live like normal people, reading and lazing around on the bench.
It's sad that housing prices have skyrocketed to the point that it's jarring to see people who aren't millionaires living in these places. I mean, everyone knew the people in
Friends
couldn't really afford those apartments, but it was sort of an acceptable "television stretch" back then instead of an absurdity.
For William, skimping by on his bookshop income, I just tell myself that's why he put up with a sloppy, annoying roommate. (I was a little worried about how much cleaning that bathtub was going to need before William could plunk Anna into it. Spike didn't seem the type to concern himself with leaving a bathtub ring.)

Regarding your last point, it is a long-standing technique in movies and TV shows where "poor" characters live in places that in real life they likely couldn't afford. I suppose they do that because it wouldn't be a very attractive home if they presented it the way it might really be in real life. Anyway that is what I always figured.
.... TxMike ....
Sometimes I think we're alone in the universe, and sometimes not.