Consumer Prices as advertised

Does anybody know which legislation says that for consumer goods, the prices advertised must be stated inclusive of VAT or at least state in small print "+ VAT".

The reason I am asking this is because Tesco recently advertised unlimited broadband package for £2.50 per month but it did not state that this price is exclusive of VAT.  Therefore, the actual price for a consumer is £3.00.  Now on top of this you also need to add the line rental of £13.75 + VAT.  Again the line rental was simply stated as £13.75 per month.  It did not state anything like "+ VAT".

The only reference for this is at this link: http://www.hmrc.gov.uk/vat/sectors/consumers/basics.htm but it does not give any statutory reference.

BT prices are always inclusive of VAT; TalkTalk prices are also inclusive of VAT.

I migrated from TalkTalk to Tesco but now it has worked out more expensive than the package I had with TalkTalk.  To get out of the contract there is a stiff penalty of making up the lost revenue to Tesco of upto 1 years package.

Thanks to anybody who can reply swiftly.