CGT rate/amount on UK non-res disposal of UK property - how does it interact with UK personal?

Hi, I wonder if someone can help with this question. I have rounded the numbers for simplicity. I am a UK non-resident and have disposed of a UK residential property. I have no UK taxable income and am entitled to a full UK personal allowance. If my taxable capital gain (so after CGT allowance etc deducted) is £15,000, do I have to pay 18% CGT on the whole £15,000 or 0% on the first £12,570 and then 18% on the remainder. I have a horrible feeling I am going to be told I have to pay 18% on the full amount. That seems a little unfair? I see if you haveincome plus a gain that takes you from the basic rate band to the higher rate band you pay 18% on the amount left in your basic rate band and 28% on the rest. Shouldn't this marginal approach apply from 0 to 18% too? Is it really the case that if you have no UK income but a gain of say £5k on a property you have to pay 18% of £5k? I'd be really grateful for any help! Many thanks..

Hi John,
You are correct in that the gain, minus the annual exempt allowance would be taxable at the lower rate (18%) up to the equivalent level of the basis rate band (£37700 in 2023 to 2024).
Any amount of gain above that level would be taxable at the higher level (28%).
Personal allowances are not a factor in calculating Capital Gains Tax.  
You would pay Capital Gains Tax on £15000 at 18%.
You have 60 days from the disposal date to report and pay the Capital Gains Tax, to avoid penalties and interest.
If you can obtain a government gateway user ID and password, you can register for a capital gains account at:
Capital Gains Tax
You may need to contact HMRC to do this.  
Your alternative is to download, print and complete a paper version and send it to:
Pay as You Earn and Self Assessment  HMRevenue and Customs  BX9 1A.
Thank you.