In short, the answer is no. While it is possible to buy a voucher for all of your shopping in Tesco, it is not possible to do so using a single voucher. However, there are a few ways that you can get around this.
The first way is to use multiple vouchers. You can use as many vouchers as you like, as long as they are all for the same value. So, if you had two £5 vouchers, you could use them both to get £10 off your shopping.
Another way to get around this is to use a voucher that can be used for multiple transactions. These vouchers are usually valid for a certain period of time, and can be used multiple times within that period. So, if you had a voucher that was valid for two weeks, you could use it once a week for two weeks, and get £10 off your shopping each time.
Finally, you could also try using a voucher that can be used across multiple shops. These are less common, but they do exist. So, if you had a voucher that could be used in Tesco and Asda, you could use it in both shops and get £10 off your shopping in each store.
So, while it is not possible to buy a voucher for all of your shopping in Tesco using a single voucher, there are ways that you can get around this. By using multiple vouchers or a voucher that can be used for multiple transactions, you can still get the discount that you want.