The Offences Against the Person Act 1861 (which includes Section 18, under which you're charged) covers the whole of the UK. Unless there are very exceptional circumstances, judges are obliged to sentence first-time offenders, convicted after a trial, in accordance with the table on page 13 of this document:
http://www.sentencing...inst-the%20person.pdf
Offenders who plead guilty at the earliest opportunity can have their sentence reduced by up to one third.
So, if you're convicted under Section 18, the minimum sentence you can expect is one of 2 years imprisonment (of which only 1 year would actually be spent 'inside').
However the assault you've described doesn't come anywhere near to the definition of 'GBH with intent'. See paragraphs 45 to 67 of this document for the definitions of 'ABH', 'GBH' and 'GBH with intent':
http://www.cps.gov.uk...s_against_the_person/
If your description of the assault is accurate, the appropriate charge should be 'ABH'. If so, the table on page 17 of the document in my first link will apply (again with a reduced sentence for an early guilty plea). In that case a custodial sentence is extremely unlikely.
Chris