the option to leave and return to be interned in the UK?
Supposing what you say is always true, which seems a little unlikely to always be the case, what access to info about ISIS do those having to make that decision have? And how much option have they actually got to leave, if that involves crossing into either government held territory or crossing territory held by other rebel groups / ISIS territory?
I expect that most of the groups who've allied to ISIS in this way have done it largely to avoid fighting on 2 fronts, so they can focus on fighting the regime / retain control of their area, rather than out of a desire to join ISIS in their attacks on other rebel groups, genocide etc. Once they've joined though, they probably will then be expected to support attacks they have little choice over supporting.
It's a hugely complex situation, and I can well see that there will be people returning who've simply got sucked further and further into an out of control situation who don't support ISIS's extremist vision or methods, but found themselves nominally fighting for ISIS. I can't remember where I read it now, but I'm sure I read about a group of a few hundred who'd attempted to leave ISIS as they didn't agree with their methods, but were rounded up and shot rather than being allowed to leave.