[TeMPOraL]

Not sure about 1800's London, but for those earlier dates, the typical community you'd be visiting was small enough that everyone knows of most members, so you'd be discovered no matter the outfit, if enough people saw your face.

The tip from 'bryanrasmussen is good - dressing as a priest is probably the safest bet; alternatively, as a soldier (in Rome) - the few people who had recognizable uniforms and were commonly traveling outsiders.