The first test for either a Section 20 (GBH) or Section 18 (GBH with intent) charge is simply the level of harm caused. (It's the same for both charges). Such harm must be 'really serious', with the Crown Prosecution Service's guidance to its staff giving the following examples of what constitutes 'GBH':
"injury resulting in permanent disability, loss of sensory function or visible disfigurement;
broken or displaced limbs or bones, including fractured skull, compound fractures, broken cheek bone, jaw, ribs, etc;
injuries which cause substantial loss of blood, usually necessitating a transfusion or result in lengthy treatment or incapacity;
serious psychiatric injury"
To add 'with intent' to a 'GBH' charge, there must be evidence of "intent to do some grievous bodily harm, or . . . intent to resist or prevent the lawful apprehension or detainer of any other person".
The CPS guidance states:
"Factors that may indicate the specific intent include:
a repeated or planned attack;
deliberate selection of a weapon or adaptation of an article to cause injury, such as breaking a glass before an attack;
making prior threats;
using an offensive weapon against, or kicking the victim's head."
With regard to 'attempted GBH' the CPS guidance states:
"It is not possible to attempt to commit a section 20 GBH offence. An attempt to cause grievous bodily harm should be charged as an attempt section 18 because, as a matter of law if a suspect attempts to cause really serious harm he must necessarily intend to do so. Similarly if a suspect attempts to cause a serious wound of a kind that would clearly amount to grievous bodily harm the offence would be attempted section 18."
See here:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a15
When sentencing such offences, judges must consider both 'harm' and 'culpability'. If both are rated as 'low' it's a Category 3 offence. With one factor rated 'high' and the other 'low' it's Category 2. If both both are 'high' it's Category 1.
Unless there are very exceptional circumstances (which the judge must explain to the public in open court), judges are compelled to sentence offenders as follows. (NB: The sentences apply to first-time offenders convicted after a trial. A guilty plea at the earliest opportunity can see sentences cut by one third but previous convictions, especially for violence, can push sentencing higher. The actual time spent in prison is normally half of the nominal sentence):
CATEGORY 3:
GBH: 'Starting point' (from which the judge must work up or down) = A 'high level' Community Order. Minimum = A 'low level' Community Order. Maximum = 51 weeks custody
GBH with intent: Starting point = 4 years. Min = 3 yrs. Max = 5yrs.
CATEGORY 2:
GBH: Starting point = 18 months. Min = 1yr. Max = 3yrs.
GBH with intent: Starting point = 6yrs. Min = 5yrs. Max = 9yrs.
CATEGORY 1:
GBH: Starting point = 3yrs. Min = 2½yrs. Max = 4yrs
GBH with intent: Starting point = 12yrs. Min = 9yrs. Max = 16yrs.
See here:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf