From Tesco's grocery T & Cs,
"Availability
If for any reason beyond our reasonable control, we are unable to supply a particular item, we will not be liable to you. Please note that we will attempt to deliver substitute lines should selected lines be unavailable unless you request us not to do so.
Although we will always try to cater for your orders, an order of large quantities of a line can only be fulfilled at the discretion of the store. For example, if you were to make an order of 10 bottles of wine which would use up all that store's stock, the store may only deliver 5 bottles.
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Acceptance
Your order is an offer to buy from us. A contract is only formed when we notify you that we have despatched your order or notify you that your order is available to collect at the store (as applicable). At any point up until then, we may decline to supply the goods to you.
The Tesco.com grocery service is available for non-commercial and domestic use only. We reserve the right to refuse orders from businesses or that we consider are for commercial or other non-domestic concerns.
To ensure availability of all our products customers may be limited to a maximum number of items."