Sainsbury’s complaint: false advertising

I was helping my mother, an OAP, to do her online shop and whilst buying some items, we noticed there were promotions available, so purchased these products, when we came to the end of the shop, we pressed on the checkout only to be told these items were not on promotion, we still purchased them as they were needed but were obviously disappointed and felt we had been tricked. 2 days later we went into our local megastore and noticed the items were all on promotion in store. On returning home we phoned the online centre and spoke with a customer service advisor, he explained that the items were not on promotion in the store our shopping comes from, after some prodding it turns out this isn’t a store but a warehouse in Bromley, our local store is in Haringey. I enquired as to which ‘stores’ the promotion was available but the customer service person refused to give me that information, he kept repeating that because when we reached checkout and it said these promotions weren’t available, that that was fine and legal on their part, I explained that if I was in store and saw a price advertised on a product, yet when I reached the till the staff member told me that price wasn’t available and it should be more, I would kick up merry hell and expect the advertised price to be honoured as to not do so is to falsely advertise a product.