In this country we do not give people a ticket to work on just one sort of gas appliance, in this case just boilers (for the obvious reason that you would be tempted to work on things that you should not or would carry out Land Lord Cert's). Every engineer has to take & pass the CCN1, core gas safety assessment, first before they then do the further assessments for the areas of gas work they wish in.
The CCN1 assessments covers a whole range of safety related issuers across a range of the basic equipment found in domestic property's, such as boilers, fires, cookers it covers flues, ventilation, pipework installation & sizing, testing, purging to name just a few. The theory knowledge can be taught in a training center, what can not is the experience to be able to apply this in the normal working situation, so that you are able to safely, confidently & quickly complete the one day of practical assessments that you have to do to pass the CCN1.
This is why you are required to work along side a GSR engineer for hundred(s) of days to practice these skills & this is how it should be thought of (practical onsite training) not just some hurdle to overcome by getting somebody to sign it off !!
Do you think they would let (or would you want to) loose around gas systems after just doing a sort course in a training center ?
As has been said on previous posts even if you pass the ACS exams / assessments you can only apply to join the GSR & will remain on probation until you have been inspected & checked on site that you are safe.
I hope this gives you a better idea of what is involved in working with gas, even if it is just combi boilers.