PedroMendez
Acolyte
The UK has stricter rules than most other nations on punishing viewpoints, so she'll be pretty likely to get judicial punishent even if no victims can be located (if they exist). Alex Davies got sentenced to 8.5 years for membership in National Action, and is I believe just one out of about 20 convicted people from that single group. While they as a group actually did some stuff, like stockpiling weapons and going on marches that turned violent, I'm pretty certain most of them were convicted for membership rather than any specific action or incident.
I fully agree with you, that the UK should take her back and prosecute her. Not doing so violates core values of western societies and exacerbates the problem of a lack of justice for the victims of ISIS. Its also a slippery slope for further infringements of civil rights. Additionally forcing foreign counties, who are already struggling, to deal with these people doesn't sit well with me either. Overall there are many reasons to be angry about the UK government.
I don't know UK laws in this regard, but what you are describing highlights one of my concerns. She might be prosecuted and go to jail, but the local victims won't be part of this process.