Tesco can choose to not serve you with alcohol due to your age, which they specify.
Put all your shopping through, then the alcohol. If refused just walk off and leave the lot there for them to sort. It won't get you the alcohol but it will cause them a little problem for a few minutes.
And, possibly, a ban.
If they ask why you are leaving it tell them "You chose not to serve me alcohol so I changed my mind and decided I didn't want the goods"