I really don't know much about AGSM or Melbourne - I assume you're in Australia right now and planning on settling down there post-MBA?
As for UK schools, you can shoot for LBS as your stretch, and Oxford, Cambridge as sweet spot schools where you should be competitive. Maybe Cranfield as a safety.
The GMAT isn't the be all end all, but a low score can significantly handicap you. For the schools you mentioned, a high GMAT score can certainly help too ("high" in this case would be 740+). A general rule of thumb with GMAT scores is that so long as your score is within their averages +/- 40 points, you're safe, and ideally at their average or above. You should be able to find the average incoming GMATs on all the schools' respective websites.
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