nailit,
No they don't, as they already have it.
The Association of Chief Police Officers(ACPO) has decreed that an Officer can deploy Taser if s/he is either faced with serious violence or the threat of it and the use of it would either prevent that violence being carried out on the Officer, members of the public, or the person(s) causing or threatening it.
Just imagine the situation: An Officer is suddenly threatened by someone brandishing a knife. By the time permission was sought, and ultimately given, you'd probably have been stabbed already.