It may be the case that the power of attorney doesn't include the right clause, as Lawbird points out, or it's just that your lawyer might not want (or have the time) to queue up at the police office for hours to have your NIE Number arranged. But you certainly don't have to travel to Spain to apply in person.

Another possibility is that you apply through the Spanish consulate in the UK. However, I don't recommend this option, especially if you are in a hurry to complete, as it may take up to 6 months to get your NIE Number delivered.