I don't have any technical knowledge on boiler servicing but, as a householder, the difference between a BG service and one done by my local heating engineer is very noticeable.
BG took the cover off the boiler and looked inside but never dismantled anything, they sometimes stuck a probe in the flue outside, they never went to the gas meter, they did stick a compartment sticker on it and left some paperwork. They often tried to sell me a new co2 alarm to replace the one that they originally sold me which still tests ok. They usually spent more time drinking coffee and updating their laptop than working on the boiler.
The local heating engineer takes off the cover and dismantles some things inside and cleans them, he flushes out the plastic bits that catch the condensate, he cleans the magnetic filter, he connects a gadget with some pipes coming out of it to the boiler and goes to the gas meter (no idea what he does there), he checks the concentration of inhibitor in the central heating water. He tests the co2 alarm. He leaves some paperwork and completes a some guarantee paperwork in the back of the boiler manual, he also leaves me a printout from the gadget that he connected to the boiler. He is usually here about 45 minutes. The only extra that he has sold me is some inhibitor when his test indicated that the concentration was low.