JD Weatherspoons - Absurdly low prices and competitive advertising

I was at our local weatherspoons, the William Webb Ellis, the other night, and they had an interesting sign on the counter comparing their prices to local pubs. This was all very interesting as some of the prices weren’t just less than others, they were a half for even a third.
Now I don’t drink redbull and vodka, but a double smirnoff and redbull for £3 is awesome, especially the rice elsewhere was £7 or more, and i can remember paying at least $10AUD at clubs for a single back in Perth.
When does this type of advertising become slander though? In this age of law suits and rules about vendor verdicts, why can a store hang a sign stating how much better their prices are than competitors? But i guess it happens more than i first thought.
Tags: Drinking Culture