First of all, you need to find what similar properties in the area are actually selling for. Some sites will give this information for free if you google around. It is not impossible that, if the price was £85k in October, there may be a similar property nearby on the market at £70k in which case you could have a long wait for a buyer at £80k.
It is possible that, if you look for another mortgage, they will ask for a deposit of 20% - 25% minimum for a "buy to let" property. This may or may not be a problem but my feeling is that, if you've already got a good mortgage in place and if they will continue with the permission, you should stick with that. Yes, getting a mortgage without being in the UK will be difficult. Without a UK address as well, it will be close to impossible.
If there is no time pressure for a sale, it is likely to benefit you in the long run to rent for a couple of years before looking at selling. This is just based on my view that the market is already depressed and while one or two sellers are lucky enough to find an enthusiastic buyer, there wont be enough good buyers to make selling as good an option as renting for a couple of years at least. Also, if you are putting it back onto the rental market in March, you are likely to get a better tenant if it is likely that they can stay there, with some certainty, for a decent period.
It is worth you asking around about the implications of taking work in Australia. It is possible that a job offer could lead to you being able to change your visa. It's definitely worth talking to people as I suspect that looking for work wont be seen as a "crime" but it's certainly worth making enquiries.