The absolute minimum sentence for a 'Section 18' (GBH with intent) offence, where the offender pleads guilty at the first opportunity, is one of two years imprisonment. (That's a nominal three year sentenced reduced by one third. The actual time spent 'inside' would normally be one year).
However, in order to obtain a conviction on that charge, the Crown prosecution Service would need to show:
(a) that the victim suffered really serious injuries (otherwise it's not 'GBH') ; and
(b) that there was a clear intent to cause such injuries. (The use of a weapon, such as the jug, can be seen as a strong indicator of such intent but it doesn't automatically push the offence up from 'Section 20' to 'Section 18').
Given that the CPS have no medical or photographic evidence to establish that the victim suffered a 'serious' injury (such as a fractured skull or a loss of blood requiring a transfusion), I can't see how they could possibly hope to convince a jury (beyond reasonable doubt) that the victim was seriously injured. If that's the case, you can't be convicted of 'GBH' (and the 'with intent' bit falls by the wayside, as it only applies to GBH cases).
If your solicitor is doing his/her job properly, he/she should be seeking to get the charge dropped to 'ABH'. The use of a weapon would probably see the offence classed as 'Category 2'. Although the 'starting point' sentence for such an offence is 26 weeks custody, an early guilty plea and evidence of good character is likely to see a Community Order imposed instead.
See here to read about what constitutes 'ABH', 'GBH' and 'GBH with intent':
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/
(That's the actual guidance document which the Crown Prosecution Service issues to its staff).
See here to read about sentencing practice:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
(That's the actual document which judges are obliged to refer to).