At the lowest end of a 'GBH with intent' offence (i.e. 'Category 3') then sentence has to be between 3 and 5 years imprisonment. At 'Category 2' the range is between 5 and 9 years. At 'Category 1' it's between 9 and 16 years.
You've probably described a 'Category 3' offence.
See pages 3 to 6 here:
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
An early guilty plea can see a third knocked off the sentence but it can't see the charge reduced to 'Section 20'.
The factors adding 'with intent' onto a GBH charge (taking it from 'Section 20' to 'Section 18') are indicated here:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a16
There might be a better chance, though, of getting the charge dropped down to 'ABH', as the injuries you describe might not be regarded as falling into the 'really serious' category, that's required to sustain a GBH charge:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a15