Why does UK use the pound and not the euro?

Because since the inception of the euro, successive UK governments have always considered that the advantages of joining are outweighed by the disadvantages

A highly political question with as many answers as there are people. Some might say that it is a bad idea in principle to lose control of the currency being used in your own country, to people who do not live there and who have different targets and ambitions which are unpredictable, and uncontrollable. Some countries are even now discovering these factors to their own cost. It is a high-principled idea, but brings many "unintended consequences", as indeed, opting out of the euro does as well.