When I did it (not very long ago) it was all over in 4 weeks, including half a dozen appliance tickets.I dont know how they can justify giving someone corgi after only 30 weeks, I think its a joke.
Can't blame the learners for not knowing what sizes the bits are, it's the HSE who set the rules on who's "safe" and what they need to know.
I asked all sorts of dumb questions early on, and upset a load of codgers who didn't like it being pointed out that there's nothing much difficult about gas, you just need to find the information. I learned masses from forums like this one. Try Gas-news, there's a newbies section there, and join Argi, where you 'll get insulted for not knowing things as well as help!
Answer to your question is that it can be difficult to cap some appliances off without sometimes cutting into the plumbing. And you do have to be ruthless sometimes. Eg with a hob with no access to an isolator tap, and one ring going out when you turn it down instead of staying alight (ie unburnt gas escaping). That's Immediately Dangerous and you have to isolate. No access, means cap off at the meter. So they lose CH and HW - sorry, it's the law. (You do need their permission, but you can threaten Transco and the police..)