While GBH is the more serious offence, and usually receives a greater punishment, the maximum custodial penalty for both offences is actually the same. (5 year's imprisonment, or 7 years if the offences was racially aggravated). The only exception is when "with intent" is added to a GBH charge. The maximum penalty is then life imprisonment.
For ABH sentencing see page 17 of this document:
http://www.sentencing...person_accessible.pdf
For GBH see page 15. For GBH with intent see page 13.
Chris