If you came from the UK, what you may not know is:

If you worked between April and your move, you would have paid income tax on each pay period as if you were going to see out the full year. The great news is you will be owed money in back taxes as your UK tax free allowance will be a greater percentage of your earnings compared to a full 12 month period.

Depending on your experience with HMRC you can claim online, through self-assessment or through a postal form.

I gave a close relatives postal address to my former employer and HMRC for all mail post move to Aus.

Your P45 can take a while to arrive in the mail or on HMRC online, but once it does you can claim a refund via form P85