The CAA aren't stating you have to carry an extra 20 mins fuel they're just saying it would be a good idea to carry it unless you fancy a diversion. I've always believed that AIC was aimed at the infrequent visitors to LHR. The local operators are more than capable of reading the METARS and CTOTs and making an educated guess as to when LHR is going to be busy and when it isn't. At a guess I'd say the money saved by no carrying the extra 20 mins more than pays for the occasional diversion.