best way of getting someone to tag along with is to meet plumber/gas fitters and offer them the jobs you can get on the promise you can tag along too in return of no pay. plenty will take what you have and cant refuse you.
find whos willing first then take any boiler services-new installs-cookers heating systems etc then co-liaise with other guy to book it in.
they in return may then let you tag onto some of their jobs giving them a hand when the working relationship gains some trust.
what you will find is not many want you to slow them down so you have to give more than you get, its not unknown for them to charge £200 a day to train you just for letting you tag on! if you can get a cut or £50 a day then your doing well!
the other implications are that you will not be insured public liability or personal- so you can see why its hard to find someone willing.