Inaction on the part of the UN (who, despite the vicious intervention by Russia and China, could have done far more than they have
Wrong.
Nothing can be done by UN against a veto.

Britain not to become practically involved because of (in their own words) the possible alternatives to Assad,

"own words"?
Wrong. No ministerial announcement had those words.
I myself have posted several ministerial statements of actual practical action that has been and is being taken by Britain.

The EU oil embargo of which Britain played a part has cost Assad four billion US dollars and halved the value of the Syrian pound.