It might have been BBC. It was a mini-series, I think. A long time ago - maybe 15 years ago? Maybe more?
I thought it might have had Polly Walker (from Bookish) and Adrian Scarborough in it, but I'm notoriously bad at this so I may be wrong on both counts.
What I do remember is the plot: Ordinary middle class family. Mum pulled in 100 directions looking after kids, husband, wider family etc. I think it begins at a celebration - maybe birthday or anniversary? And THEN it transpires that the husband has had an affair and the main character, the woman, goes sort of eerily calm and takes stock and starts living her life for herself. The story is about her putting herself first, and how the family struggle without her putting them first.
Shot in a British city (maybe London).
It was very good, and very popular.
Any bells?