Let's lay out the facts, shall we?
She's a British citizen. You can't revoke that citizenship without making her stateless and violating treaties we've ratified in the process. Being a British citizen, we can't block her coming back (unless we use certain other powers like Temporary Exclusion Orders).
She also deserves the right to a fair trial. If you don't give one citizen a fair trial, you might as well stop calling it a universal right.
Not to mention that her child, who is about to be born, has British citizenship as well, and hasn't done anything except have bad luck with their parentage.
However, with IS in decline, they're likely to adopt more 'traditional' methods of attack. Integrate militants back into their communities, forming undetected autonomous terrorist cells. She's a threat, plain and simple.
I'm not an expert, but terrorism does fascinate (and terrify) me. Here is my semi-informed suggestion. If she can make her way back, accept her into the country, detain her, investigate her, and ensure there are no immediate threats. When that's done, try her for membership of a proscribed organisation - that's five to ten years right there. You could add attendance at a place used for terrorist training; under the Terrorism Act 2006 that's another ten years. Take the child away, adopt it out or make it a ward of court. Enforce an order so that the child will never know who their mother is (cruel, perhaps, but necessary).