Almost all the handling agents at UK airports (e.g. Aviance, Servisair, Airways Handling, etc.) do their own training in-house but will train people outside thier companies for a modest fee.
Instructions for towing an aircraft are in the Aircraft Maintenance Manual.
You can search the CAA website for publications regarding push-back and airfield operations, you might find something there.
The airfield operator (airport) will have its own instructions/requirements as well.