When I went through the NATS application system, any criminal convications were only relevant to your security clearance, not to the main part of the application that HR considers on application. Indeed the criminal record part was detached from the main part of the form and submitted in a sealed envelope supposedly for the security vetters' eyes only.
I have heard that the main issue for vetters is that you own up to any convictions as they frequently check up. As far as they are concerned a person who is hiding major secrets from their employer is a person is more easily blackmailed by an outside party, which is more of a security risk than employing a person with a minor criminal record who is prepared to 'fess up.
(I have a minor spent caution from ages ago blotting my otherwise clean copy book and they let me in; plus this was a while ago back in the days of paper application forms!)