I'm not overly familiar with Constantius' reconquest of Britain, but I can more or less answer your question by referring to Rome's general policy regarding the reconquering of lost territory.

As a rule of thumb, no, they did not enslave or sack the towns of rebellious Roman populations. I emphasize Roman populations though, since rebellious tribes that had once submitted to Rome were rarely extended the same courtesy. During the Jewish Revolt, for example, the Romans systematically sacked Jewish settlements and enslaved or massacred their inhabitants, even razing Jerusalem itself to the ground. Conversely, at roughly the same time, during the Civilis Revolt, most rebellious Roman towns were not sacked, and allowed to pledge their allegiance to Rome once more, peacefully.

It largely depended on whether you were a recently conquered people, or if you had culturally integrated and were a citizen of Rome. In the case of the former, you could expect swift reprisal after rebelling. In the case of the latter, chances are you'd have seen mercy. Since Britain in 296AD would have, for the most part, no longer have been tribal and had more or less integrated into Roman society, it is unlikely that much of its population would have been enslaved or seen significant reprisal.

See Stephen Dando-Collins' Legions of Rome, if you're interested in further details regarding the subject matter.