A contract is formed when an offer is made (you saying I'll give you £5 for those drinks or them saying give me £5 for those drinks) The other party accepts the offer and there is some 'consideration'.. in this case, money, handed over..
What you are describing is 'invitation to treat.' It forms no basis of a contract.. very similar to you going into Curry's and they have put a price of £10 on a £10k TV.. they are not obliged to sell it at that price as it is not an offer, the offer is made when you say, I'll give you £10 for that tv... and they say NO!
Doesn't matter what posters are on the wall or what time it is... it is all 'invitation to treat,' the offer is made when you offer to pay a certain price for the drinks and they are quite within their rights to turn down that offer. In a situation such as a public house, the law would hold that it would be a fair assumption that you should know how much the drinks were about to cost before you ordered them. That cost will be based on full retail price as the adverts do not constitute an offer.
By the same respect, you could have refused to accept the drinks as at that point no consideration had been made.