Capital gains from sale abroad

I plan to return to the UK in September after 9 years abroad. My house (main residence from the last 3 years) is already under offer. In the scenario that I move back and my income is paid in UK from 1st September and the property sale completes on 1st October: - From which date would I be deemed resident for UK income tax? - Would I be liable for Capital Gains tax on the sale of the property? and if so how would this be calculated? Thanks for your help

Hi,
If you are resident in the UK, when you dispose of the property, you may have UK capital gains tax to pay.  Whether a gain arises or not, the disposal must be declared an a self assessment tax return.  
You would need to work out if there is a gain, using UK capital gains tax rules. There is a calculator at:
Capital Gains Tax
All parts of the calculation must be in pound sterling, using a just and reasonable exchange rate in use at the time of acquisition and disposal. Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal.  
For your convenience, there are exchange rates at:
National Archives: Exchange rates from HMRC in CSV and XML format
For older rates at:
National Archives: Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009
You are free to use any of the supplied rates or one of your own choosing. 
Thank you.