...there is a law against this!
It is an unfair commercial practice according to the nattily titled "Directive 2005/29/EC of the European Parliament and of the Council concerning unfair business-to-consumer commercial practices in the internal market" (also known as the ‘Unfair Commercial Practices Directive’). This is implemented into UK law by The Consumer Protection from Unfair Trading Regulations 2008.
In particular, Schedule 1 para 5 provides that "Making an invitation to purchase products at a specified price without disclosing the existence of any reasonable grounds the trader may have for believing that he will not be able to offer for supply, or to procure another trader to supply, those products or equivalent products at that price for a period that is, and in quantities that are, reasonable having regard to the product, the scale of advertising of the product and the price offered (bait advertising)" is considered automatically unfair.
It's late, I can't sleep, but I don't feel the need to set out the consequences of unfair trading. Knock yourselves out... http://lmgtfy.com/?q=The+Consumer+Protection+from+Unfair+Trading+Regulations+2008+(No.+1277)