Welcome
adelaide is a fairly limited market for international, essentially you’re looking at Emirates, possibly Qatar or Cathay pacific if you wanted to literally depart Adelaide and arrive Manchester. Malaysia airlines may be able to sell you a 2 stopper with a second change at healthrow as an alternative
have a play at around with google flights-
https://www.google.com/flights?hl=en
you can make it flexible on dates and also specify alternate entry points eg it may be cheaper to get yourself to Sydney on any old airline and take the international flight out of there, or alternatively fly into another UK or European port and take a short connecting flight to Manchester- this would potentially be on a separate ticket in which case you would need to save plenty of time
i think you can set up price alerts on google flights as well but not sure
i’m not aware of any current sales although I think Qatar had one recently (i’m not certain if they fly into Manchester but think they do), qantas usually has a sale around mid December
hopefully someone with a bit more knowledge will come and talk about advance purchase rules- essentially buying the ticket ASAP is usually a bit pricier and there is a sweet spot where prices drop to “relatively low” around 6-12 weeks before the flight- i’m not sure when that is for business ex-Australia as my booking patterns don’t align to it usually
you can also keep an eye on the premium fare deals forum which has helpful stuff from time to time
https://www.flyertalk.com/forum/premium-fare-deals-740/