Breach of the Sunday trading laws, large stores can not open for sales until 10 am. 'Browsing time' is a loophole that they have exploited to increase profit, you can look round and fill up your trolley but it is illegal to actually buy it before 10 am.
You are also guilty (technically at least) of theft as the store can not accept your offer to pay until 10 am.
But basically you 'bought' an item when it was illegal for the sale to take place. If this actually happened both you and the store would have committed an offence.