ABH is the less serious of the two, it is assualt which resulted in minor injuries for example bruise.(5 years max)
GBH is serious harm to a person i.e broken bones, split skin and is split into 2 sections section18 and section 20.
From Section 20 of the Offences Against The Person Act 1861.
I quote "Whosoever shall unlawfully and maliciously wound or inflict any grievous bodily harm upon any other person, either with or without any weapon or instrument, shall be guilty of an offence and, being convicted therefore, shall be liable to a term of imprisonment not exceeding five years."
This differs from Section 18
"Whosoever shall unlawfully and maliciously by any means whatsoever wound or cause any grievous bodily harm to any person, with intent to do some grievous bodily harm to any person, or with intent to resist or prevent the lawful apprehension or detainer of any person, shall be guilty of an offence and, being convicted thereof, shall be liable to imprisonment for life."
difference between section 18 and 20 is the intent to do cause GBH.