Theoretically 'plea bargaining' (in the American way) is prohibited within the English judicial system. However, as you seem to realise, a great deal of discussion often goes on between the CPS and defence counsels, in order to seek 'common ground'.
Based upon the content of your other post, I can see little prospect of getting the Section 18 charge down to Section 20. The CPS website allows you to read an internal document which advises prosecutors on which charges they should bring. While the document is not, in itself, a statement of the law, it's based upon decisions which have previously been reached by the courts. It would require exceedingly unusual circumstances for a court to overturn a legal precedent so, in effect, the document is a fairly accurate statement of the law:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/
The section starting at paragraph 50 of that document defines GBH and cites examples of the types of injuries which can lead to such a charge. (You'll see, for example, that a fractured jaw is mentioned there).
The section starting at paragraph 59 defines the additional factors which add 'with intent' to a GBH charge. You need to read through it and ask yourself whether your actions meet the criteria given there.
There's probably little point in seeking to change your barrister at this stage. However I recommend speaking to your solicitor and telling him (or her) that you want a 'conference meeting' (at which you, your solicitor and barrister will all be present), as soon as possible, in order to consider your options.
Chris