How capable were British militia units in the Napoleonic era?

Were British militia mainly a low-risk hobby for the gentry and a force for suppressing riots?

That describes part of their role, but quality varied. Some units were well trained, some went abroad as regiments, and others trained recruits for the regular army.

Militia quality varied, so the image of a gentleman's hobby or a force used only against riots is incomplete. Yeomanry and some other units were well trained, while prolonged full-time service could make embodied militia proficient soldiers. Some militia units became regiments and went abroad. The militia also served as a recruiting and training base for men entering regular service.