Most companies don't have a "system" for employing people without paying them therefor approaching through the "front door" via cold call or letter to HR will probably not work.
Going through a Maintenance Manager of Senior LAE contact would be better or perhaps calling some of the recruitment agency'c that can "get you in" would be better (as they won't get any money - they may not be that interested other than a bit of credit for finding a free labourer).
You probably won't get any line work atall as having untrained people on the ramp is a bad thing - aiming for hangars would be better.
I would have thought Flybe at MAN would let you into their "A" Check hangar to play.