BUT I don't think that story would necessarily have fitted into this series.
Why not?
The other 2 films dealt with issues which aren't often discussed or portrayed on TV. Whereas the whole 'body clock ticking/why can't a beautiful, clever, succesful woman find a man' thing has been done SO many times! And while I know not everyone agrees, IMO this one wasn't particularly original or interesting in its portrayal. Just one cliche after another.
I don't see why Gemma Chan couldn't have portrayed a child-free woman fending off the same overbearing mother's (yet another cliche of this type of story) demands that she reproduce, or dealing with friends asking her why she hasn't 'settled down' and she'll regret it later blah blah blah. Would have been more interesting - at least for me - than yet more tears at holding a baby and dancing around the flat on your onesome.