Unfortunately although the heating trade is ridiculously busy at present and I am, along with many others, desperate to find someone to employ, unless you have a large firm with many employees already you have not got the time needed to spend on nurturing a trainee in the ways you need them to operate.
I would suggest contacting the larger firms, maybe new build installers where you can gain useful, but boring & repetitive experience and build up your speed as well.
The down side to new build is that once tarred with this brush, domestic installers like myself would never employ a 'housebasher'
Does not your training centre help with placements, or are they only after your money?
Good luck and keep trying.
.so do you want to be a "proper" plumber.ie. copper work....iron pipefitting ....(welding).....drainage....(iron ,plastic, above +below ground).... water supply regs....guttering, sheet metal (lead etc)....or do you want to be a C-O-R-G-I 
and stand in front of a gas boiler with a few meters etc. and leave with £100 in your pocket after 1 hour ...NO dis. to Agile 
Oh dear my hat`s stuck to my head 
