Jenny, in that period the Militia (UK wide) was the military reserve. During the 1820s Britain fought a couple of wars in Africa and Burma, but no major conflict which would require calling on the reserve. Therefore the role of the Bradfield militia would be the routine task of training locally
Hope that helps
Dave

try here,
http://www.bradfieldarchives.co.uk/?s=1&parent=262&q=262

I had found an ancestor of my son in law who was in the militia in Bradfield from 1823 to about 1830 so now I know what he would have been doing.

Thanks

Jenny