Currently, there is no sentencing guideline for conspiracy to commit grievous bodily harm (GBH). This offence is covered by common law and separate to the offences related to causing or inflicting GBH.
It is for the Sentencing Council for England and Wales, which is independent of government, to develop sentencing guidelines and monitor their use. The Sentencing Council are in the process of reviewing the definitive guideline on assault offences, and anticipate issuing a consultation on a revised guideline in early 2020.
The Assault guideline and evaluation are available here: