Firstly, your brother-in-law might want to get his solicitor (and/or barrister) to speak to the Crown Prosecution Service to see if they'll reduce the charge from 'GBH' to 'ABH'. See paragraphs 46 and 55 here to see the different levels of injuries required for each charge:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P189_14382
(The circumstances you describe seem to indicate that the decision is a borderline one).
Assuming that the charge remains as 'GBH', you've not indicated whether it's under 'Section 20' (GBH) or 'Section 18' (GBH with intent). 'Deliberate selection of a weapon' (i.e. the copper pipe) can add 'with intent' to a GBH charge. (See paragraph 65 in that link).
This document shows the sentences applicable to first time offenders convicted after a trial. (A reduction of one third is applicable for an early guilty plea):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
For 'GBH', see the table on page 15
For 'GBH with intent', see the table on page 13.
For 'ABH', see the table on page 17.
If the charge can be reduced to 'ABH' the chances are that your brother-in-law will almost certainly receive a non-custodial sentence.
However if the charge remains as 'GBH' (without intent) your brother-in-law's solicitor/barrister will have to battle hard to keep him out of prison. (Judges are obliged to follow the guidelines in those tables unless there are genuinely exceptional circumstances, which the judge must explain in open court). A sentence of around 12 months would be most likely.
If the 'selection of a weapon' sees a 'Section 18' charge being use