There is a lot of good training available. A few providers and their courses that you might want to consider are listed below. If you do book these courses, please let them know that you saw them here.
The Association of British Theatre Technicians offers bronze, silver and gold level courses which cover a range of useful skills for technicians working in theatrical environments.
The ABTT also created the TAPthis scheme to allow records to be stored and verified so that a technician can carry a small card with an ID that allows employers to quickly verify that valid training has been undertaken.
Creative Skillset offer training covering a very wide field. We highly recommend the certificate in temporary electrical systems. Originally developed on behalf of the film and TV industries, the Skillset Certificate in temporary electrical systems (SCiTES) has been created for all of those working with temporary power, particularly (but not exclusively) in the broadcast/film, theatre, festivals, shows and exhibition industries.
A range of electrical courses aimed at the entertainment industry are available. More information can be found here.