The problem is one of the principles of war. The first British principle is:
Selection and Maintenance of the Aim
A single, unambiguous aim is the keystone of successful military operations. Selection and maintenance of the aim is regarded as the master principle of war.
In the USA the first objective is:
Direct every military operation toward a clearly defined, decisive and attainable objective. The ultimate military purpose of war is the destruction of the enemy's ability to fight and will to fight.
In Britain, is the aim defeat of ISIL or the overthrow of Assad?
In the USA case their objective appears to have defined the enemy as Assad.
Russia, OTOH, is supporting Assad and thus opposed to the USA. This leaves the UK in a quandary. Do we support Obama and attack Assad or do we support Russia and support Assad?