Section 18 and section 20 are different. (obviously)
Section 18 is GBH with intent, the worse of the two. Section 20 is still GBH, but does not require intent. I.E, GBH may be inflicted unto the victim, however, to be charged wih this (S20), there need not to be specific intent to do the damage.
Bubbly, to be honest, I still don't think you make a whole lot of sense. You Say=
My partner has no previous, the other person involved has previous, he was out on licence when arrested for the offence along with my partner. Thats why they said he would get a long time, so to reduce this my partner has been advised to plead guilty on sct 18
*so to reduce this, my partner has been advised to plead guilty*
Why would you want your partner to plead guilty to reduce the time of the offender? This doesn't make sense. Or at least what you imply doesn't make sense.
Sorry but I'm not following, apologies if I have the wrong end of the stick. Also, I suggest you get a different lawyer's opinion, because pleading guilty to section 18 would seem very bad for his future!! Bearing in mind S18 can be punishable by a life custodial sentence. (although this rare). Being convicted of a section 18 has huge life consequences. For example, I would be most impressed if he was to ever find a job again, and also, to travel to any country again outsude of the EU. (or even inside)
You would be banned for life from most countries outside the EU (if not all) if you were convicted. Don't forget, lawyers might be in it for themselves, and not acting in the best interests of their clients. GET a Different Opinion.
Best O' Luck.