You can appoint anyone who is over the age of 18 and who has not been declared bankrupt. It makes sense to appoint someone living in the UK, and preferably, someone who lives close to you. The most important consideration is to appoint someone that you trust implicitly.
Your Attorneys are of course required to act in your best interests though sadly many all too many Attorneys abuse their position. That is why it is vital to appoint a trustworthy Attorney.