As a British citizen she is subject to British law ...if she comes back Id hope the full force of the law is applied.
as abhorrent as it is, given her reported comments about no regrets etc , she was initially groomed to go off to Syria to become a jihadi bride/ sex slave to radicalised killers, some may perceive she is a victim (of radicalisation if nothing else? ), .... as she doesnt have dual nationality, under international law she cannot have her (British) citizenship revoked leaving her stateless.... however British/ English and Welsh law can be applied when she sets foot on uk soil. I for one hope she is handed a long and lengthy period of incarceration ,along with being put through a de radicalisation programme. Or even, as she could be, be put on trial for treason- for which i think there is still the option of capital punishment (hanging) for any British citizen found guilty of treason?