UK or Spain CGT

We are selling a small apartment in Spain and will incur CGT. Should I pay it in Uk first as a resident or in Spain first as that is where gain will be made?

Hi
As the apartment is 'immoveable property', Spain has the right to tax you on any gains from the disposal of the asset.  
As you will be in receipt of a foreign gain, you meet the criteria to submit a self assessment tax return.  
You would declare the gain on SA108 (capital gains) and if foreign tax was paid, you would also declare it on SA106 (foreign) so that you can claim a foreign tax credit for the tax paid in Spain, so that you avoid double taxation.
Thankyou.