So let's consider the options. Say the choice was made to bring her to the UK, how? You'd need for them to travel from Syria to a friendly nation, likely Turkey, where there is a British consulate. There is no safe road or air link so you'd need to travel overland in a truck or maybe charter a chopper, but good luck finding someone to take that contract.
For starters you wouldn't transport a deathly ill baby, no responsible doctor would sign off on that, so she's not going anywhere until the local hospital stabilises the kid which is what it tried to do anyway. The only logical time to do it was before the birth of the child and that boat was long past.
There was the option floated to return the kid but not the mother but the mother refused this as she wanted to stay with the kid. That was clearly a ploy as she would have been separated from it anyway the second she landed in London and was sent to jail with the kid going into care.
But by far the biggest reason here is the fact that to get her out you would be sending British officials into a warzone swarming with unaccounted for ISIS members. The Foreign Secretary has specifically cited this danger to life as a reason it didn't happen. You'd risk multiple lives for the sake of one, and it would be an extremely hazardous journey a sick kid likely wouldn't survive anyway.
So it is what it is, and I do not like the idea of the Govt sending British citizens to their potential deaths, and at ISIS hands they would be extremely horrible deaths, for a lost cause just to appease a section of the media. I do feel bad for the kid, but it isn't as simply as just picking it up and bringing it or the mother home. It would be an extremely dangerous mission into hostile territory that was not secure of ISIS who would just love to behead some British doctors and aid workers on the net. I remind you it has happened more than once before