Doing what - Fires, cookers, meters, pipework installations not just putting a boilers on a wall, I hope?
Do you honestly think this will provide you with enough practical experience to be able to work as a gas engineer, we don't just do the gas on a boiler as you seem to understand, so I would start the other way round like most do, learn the plumbing / heating side first before moving onto gas.
It is not the training centre you are paying that you have to convince but GasSafe or more importantly the people you will be expecting to part with their hard cash!!
You might also want to take a look at some more posts relating to gas on here, there was a guy on here posting this very morning, he is doing 10 jobs a day, 5 to 6 days aweek for £20K a year, it might not quite be the job you have been lead to believe it is ??