Could I...yes you can for what you specifically ask.
Do the Australian Class 1 medical in UK, from someone
authorised
however as has been pointed out you cannot then 'convert medicals' per say.
To save a little time, give the Doc a call, explain your situation and find out who to go to for the Ophthalmic report so you have that already when going for the actual medical.
Yes, do check the intended airlines that are your goal, some have their own requirements that can be higher than a licence medical.
Also strongly suggest you have a backup plan, sounds like you could be with few options if you don't get in a cadet scheme.
I believe the eye limitation for JAA initial medical issue is +2. Have you considered laser surgery? Also talk to the UKCAA medical unit, I believe should you return with sufficient flight hours experience that +2 might be a little bit bendable...