The Euro has been successfully used for years. There is no reason to believe that the Euro is going to fail. If for some reason, the Euro were to fail, the United Kingdom could easily return to some other type of currency. At this time, however, it only makes since to use the Euro as that is what is being used throughout the rest of Europe.
No, there is not enough flexibility in the United Kingdom to deal with the Euro, because the Euro is too strong and covers too large of an area. The United Kingdom has very strong financial institutions, and joining the Euro would make the United Kingdom too easily swayed by the financial problems in the rest of Europe.