The daft logic of paying well over the odds is that the customer gets a bigger discount.
BG market their installs based on the "saving" the customer gets through the discount given.
The principles of daft logic suggest that the more you pay for something the better it will be, so getting a larger discount by paying a lot more seems like a bargain.
Plus, because of the name Centrica (British Gas) inherited people trust the brand and assume the price will be justified by a high standard of work.
The ASA say their mission is to:-
"Our mission is to ensure that advertising in all media is legal, decent, honest and truthful, to the benefit of consumers, business and society."
[from the ASA website]
Bamboozling people with advertising is not illegal, and is the product of an industry that enjoys great rewards for legally misleading people.
The ASA, rather like Trading Standards, are organisations used to give false assurance regarding protection for the consumer. They will occasionally pick off some breach of their codes/ regulations to justify their existence (usually a small trader). When it comes to a company like BG, they are like Nelson when he put his telescope to his blind eye and said: "I see no ships!"
The rules of the jungle are: "buyer beware", whoever the company is - BG being a classic example.
I think it's fair to say that BG products don't contain horse meat, but there is quite a bit of what comes out of the back end of horses in their advertising.