Elaborate. If this war happens, and I can't express how much I don't want it to, Putin's guy Assad will be ousted/killed. What is "up to" that can stop that?
There won't be war, I hope not
The US and UK have limited options. There won't be a UNSC resolution to intervene and oust Assad because Russa/China will veto.
Striking regime targets in the air is risky because the Syrian Air Defenses are way more advanced than Libya/Iraq and they are near populated areas of Damascus, Latakia, Tartus, etc. Russian personnel are also on those sites. So, would the US want to inflict casualties on them?
Striking regime positions or damaging an military airport may happen. But again, if you strike a regime position esp if they're engaging with Al-Qaeda type groups, it defeats the purpose of "fighting terrorist groups".
Also, the US Air Force bombed and killed 100 Syrian soldiers in Deir Ezzor in September. This actually helped ISIS indirectly.