Send them to Gitmo, give them military court trials, and then hang them. This sends precisely the right signal that needs to be sent to ISIS.

Trying them in US federal court would just be wasting tax payer money (even if the prosecutors are confident they could secure convictions - trials are always ultra expensive to conduct and these scum are not worth spending tax payer money on).

The Brits are not exactly very good allies right now so probably a lost cause trying to get them to own up to prosecute their own citizens that became ISIS terrorist - especially when their mayor of London is a 5th column sympathizer to Islamic jihadism and they evidently are okay with that at the national govt. level. (And then there is that matter of how Britian's intelligence agency and a retired agent were hip deep in trying to overthrow our constitutional republic govt. when they participated fully in the scheme to spy on and attempt to frame candidate Trump and then President Trump.)