Helping the aggressor with its offense or defense during the aggressive acts is taking part in the aggression. States have international obligation to not engage in or promote aggression and to not take part in it. UK voluntarily took this obligation on itself.

I guess it's debatable whether the drone attack was proportional. I'd say that attack on clearly military installation of active ally is proportionate. Bombing bases in Britain would be more appropriate I think, since that's where the bombers that attacked Iran flew from and were loaded with weapons.