Yes they are, but that is not the focus of the advertising regulations. If you think for a minute in the context of car insurance- they can't all be the cheapest, can they?
The purpose of advertising regulations is to prevent a seller making false or misleading claims about the quality of the product- the size, effectiveness or content, for example- things that aren't apparent until after the buyer has parted with money and bought the item. It is at this point that the buyer suffers some harm, in the financial outlay that they likely cannot recover.
When companies advertise as the cheapest, you suffer no harm. The courts take the pragmatic view that a consumer will investigate and 'shop around' to find the best deal. In this context, price promises could even be seen as a positive thing in encouraging consumer knowledge and research- the profligation of car insurance comparison websites attest to this.
As you suffer no harm from such a claim, and as the claims do not go to the inherent qualities of the product that aren't discoverable until after purchase, no offences are committed here. Such an offence would be very difficult to police anyway. If Ryanair advertise (as they did) that they could take me to Oslo for £10, and with BA it would cost £150, there is no offence. Ryanair actually fly to an airport on the outskirts of Oslo, Torp- so if I actually wanted to go to Oslo itself, which air carrier is cheapest? You can see potential difficulties that arise in deciding which competitor is 'lying' here.