The security companies I have worked with previously tend to employ those from the police and armed forces (or ex-fire for fire prevention and detection systems).
Most (all?) of these organisations are currently accredited by NSI (National Security Inspectorate), although I understand that this organisation will be abolished in favour of self-regulation.
Given the role that security companies undertake, they perform rigorous searches before offering posts to people. They even need to ensure that if you have not got a continuous work history then you were doing something meaningful in that time (and can prove it).
The upshot is that I would not expect them to look favourably on you, although you may come across a more enlightened organisation (for example, many IT security companies employ ex-hackers).
The only advice I have is to approach the security companies directly. I would suggest that you try the smaller ones first, especially those that do not offer man guarding services, as these have a reputation to uphold - many of their clients would not want an ex-offender walking around their offices at night. The smaller guys tend to offer intruder detection, monitoring and prevention systems (CCTV, door access, alarm maintenance etc.), which may be more aligned to your skills.