As a Fleet Trainer, with over 12 years' experience working as a self-employed agent for four companies, I can recommend this as an enjoyable and mentally rewarding experience.
However, income can be less than for "L" driver tuition which probably involves less travelling.
You will need to have passed an Advanced Driving Test and I recommend that you contact various Fleet Training companies, most of whom offer training courses leading to the DSA Registered Fleet Trainer qualification (DSA RFT) to compare their course costs and contents, which should include both classroom presentation and on-road coaching skills.
You will first need an DSA ADI qualification and, although the DSA RFT is currently legally voluntary, most training companies require it.
I have worked for and can recommend the following national companies:
IAM Fleet, based in London:
www.iam.org.uk/iamfleet
Driving Services, based in The Wirral, Cheshire
www.drivingservices.com
I have also worked for:
ATC driver Training, based in Essex
www.driver-training.co.uk
Defensive Driver Training, based in Stourbridge
http://www.ddtgroup.com/
There are many others, e.g.
Drive & Survive at Banbury
http://www.drivesurvive.co.uk/
DriveTech at Crawley
http://www.drivetech.co.uk/
Please feel free to contact me directly if you require fiether details.
David Wornham
Flexibase Services
David@flexibase.org.uk
Advanced Training for Driver Enjoyment and Safety
Advanced Driving/Motorcycling and Instructor/Observer/Tutor Training CDs
www.flexibase.org.uk