The advertised price is only an 'offer to sell' you take it or leave it. Once you have placed the order you have agreed the price. You could try to talk down to the lower price and the company might as a gesture of goodwill reduce it - but they might not.
Some suppliers may charge the price applicable on the day of despatch, and their terms and conditions usually state that.
If the price that you have agreed via a on-line or mail order sale is a lot different to the new reduced price,then you could return the goods under the Consumer Protection [Distance Selling] Regulations 2000 within seven days. Then buy again at the new price, but you will be required to pay delivery-postage charges.