Binnz10
You need to take specialist advice on these issues; as always things are not as clear as you would imagine.
To avoid a CGT charge in the UK (ignoring for the moment the fact that both properties could in any event attract private residence relief should they be taxable in the UK), you would need to demonstrate that you have left the UK permanently. Without doing so, you will remain ordinarily resident in the UK and therefore taxable on gains arising from worldwide assets.
Once this obstacle has been overcome, and provided you have definitively become resident in Australia, then any gains will be subject to tax in Australia. However, the encouraging position here is that Australia use a rebasing system for valuing assets held by people coming into the country. This works by rebasing the assets to their market value as of the date of immigration.
That being the case, any gains would therefore be restricted to the difference bewteen the sales price and the MV at date of immigration.
Kindest regards,
John S King
Chartered Tax Adviser
www.taxation-advice.com