Why would any of them end up with a criminal record? They seem to have broken military law, but I can't see a civil offence that would follow them. Not sure how much they would have to divulge about dismissal (perhaps with disgrace?), but nowadays Army 2004-2013 on a CV seems to be the max anyone actually looks into someone's history?
Some service offences always come under the Rehabilitation of Offenders Act, with others it depends on the penalty, e.g. dismissal or a sentence of detention for (from recollection) 3 months or more.
The actual rules covering various situations are more complicated than that and are subject to change, so I suggest that anyone to whom this applies should check carefully before making any declaration - and before pleading guilty in any service proceedings including summary dealings.