Thank you for your question.
Assuming you have not lived in the property as your main residence and it is a second home (investment property) for you, capital gains calculations as follows:
Gain on sale (100,000-62,500) 37,500
Chargeable gain after gains allowance (37,500-12,000) £25,500
You state ..I am retired and pay tax at the basic rate but am close to the threshold.
CGT at 28% (worst scenario) £7,140
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