Firstly he’ll need a PTS, for that he’ll need a company / agency willing to sponsor him. There’s a Drug & Alchohol test and that’s zero tolerance as is the policy right through rail. If you get caught with any non prescribed drugs or more than 30mg/l of booze in you it’s an instant 5yr ban.
Most agencies aren’t too bothered about a criminal record, so long as it’s nothing too serious.
I use VGC, McGinley and Anders Elite to source my labour.
Contrary to popular belief there is work outside the M25, several hundred million pounds of it actually and an awful lot of it is in the north & middle of England.
Jarvis & Birse Rail no longer exist as companies directly, the former was wound up a while ago and the latter is owned by Balfour Beatty Rail and trades under that name.
Direct labour agency is the way to go in my opinion for someone of a less than Snow White background.
Not mentioning being away at Her majesty’s pleasure is illegal and will get found out anyway