I do a boiler service plan (not breakdown cover) for 5.50 per month, full strip down, parts and gaskets at extra cost. I explain to my customer that many break down plans include an annual safety check but not a full strip down. British Gas just fga it, their philosophy is they'll service it when it breaks down as it isn't worth spending extra time on it when they're receiving the same payment off the customer. Also it is more likely to go knackered as it's neglected so they can condemn it quicker and sell them a new one.
Prevention is better than cure so when did a fga suffice? The answer is when British Gas started cutting corners on their servicing and people thought it was acceptable.
Many manufacturers state that a full strip down isnt needed every year on their latest boilers, fair enough, I do these every 4 or 5 years or when the tests state otherwise.