UK property sale while living abroad - CGT backdating?

Hi. Looking to sell my uk property as I now live abroad. Bought the flat in 2006, Is it possible to backdate the value to 2015 or is it only taxed on the gains since 2015? ( I'm sure I heard it was possible to backdate the value to 2015 but the calc page mentions from 2015. Looking at land registry between 2006 and 2015 the price didnt change. ) I cannot find documentation on the costs from back in 2006, could I use average / reasonable costs for the calculation. Is there anything I am missing in terms of mitigating CGT?

Hi,
You can see guidance here:
Work out your tax if you're a non-resident selling UK property or land
Thank you.