Read through the FAQ section on here and all the messages in the Rookie section. Also have a look online about exactly what ROVs are and what jobs are done, there is no such thing as a maintainer, you are either a Pilot Technician, Sub Engineer or Supervisor.
Unlike many other industries, there are no formal qualifications, tickets, certificates or anything else required to become a Pilot Tech (despite what some trade organisations and training schools try and make you believe). We are not divers, we do not need certificates as they do.
What companies want are people who are capable of maintaining and repairing electrical, electronic and hydraulic equipment. You cannot learn to fly and ROV until you become an ROV Pilot. Many training courses give a very basic insight into what you can expect but you can never walk out of one course a fully qualified Pilot (again, there is no such thing as a qualification).
Which is why you need to show that you can repair and maintain the equipment. Can you get any equivalent national certification such as ONC, SVQ, NVQ, HNC for any work in the Forces?
Even with all the skills you may have and be highly suitable, you then have the problem that so do hundreds of others and they are all after the same handful of jobs that you are.