Hi @JLSD,
When we talk about when you can claim the offset, it's in our financial year. This does mean you might need to pro-rata the amount and spread it over two tax returns.
When converting the GBP to AUD, our rules state to use the exchange rate at the time the exchange occurred. This could be the date you converted your GBP to AUD.
You'll need to let us know about your capital gain in our financial year. This might mean you need to pro-rata it in your UK tax return.
We've a fantastic guide to CGT and property you might be interested in having a look at. It includes a link to the guide to foreign income tax offset rules.