The Crown Prosecution Service have to agree to any prosecution.
The advice to CPS staff amplifies some definitions. They also took the trouble to read through the relatively small amount of debate in Parliament.
"Although the Act does not state what a serious injury is, prosecutors must be
aware that by the very nature of its name serious injury will not include
trivial or transient injuries which include bruises and grazes."
"Taking an example which was raised during parliamentary
debates on the Criminal Justice and Immigration Bill, the anal sex scene in the
movie Last Tango in Paris, even if it were to be considered pornographic and of
an obscene nature, would not be caught by the new offence, because it is not
explicit and does not portray an act resulting or likely to result in serious
injury to an persons anus."
"The painting Leda and the Swan, another example raised
during debates in Parliament, would also not be caught by the new offence,
because it would not meet the explicit and realistic test."
"The police may ask you to consent on behalf of the DPP over the telephone. Prosecutors should not do this."