I've sat in on a lot of Crown court sentencing hearings. In a few cases they've lasted only a few minutes, leaving everyone to wonder whether the judge had read or listened to anything put before him.
However, in the vast majority of cases, the judge has clearly taken every possible care to come to the correct decision, by taking all representations into account. The last one I attended lasted 2½ hours, with the defence barrister making representations for his client and then the judge asking both the CPS barrister and the police representative for their opinions on various matters. He then asked the offender some questions, with the offender taking the opportunity to ask the judge to look at particular parts of the pre-sentence report. The judge then spent some time studying them, and again canvassed opinions and representations from those present.
The judge was clearly trying to decide whether he could find a way to avoid a custodial sentence and, eventually, that's exactly what he did. So while there's no guarantee that a pre-sentence report could help you, it would be madness not to try to use it to your advantage.
However you need to be aware that, unless there are very exceptional circumstances, judges must pass sentences for GBH based upon the table on page 15 of this document:
http://www.sentencing...person_accessible.pdf
The sentences there refer to a first-time offender who is convicted after a trial. An early guilty plea can see a reduction in the sentence. (In the case of a custodial sentence that by up to one third although, as you'll see, a custodial sentence can be avoided at the lowest level of GBH).
Chris