Settlement of Share Options paid out by company and taxed as PAYE

I retired in July 2023. I received my P45 end of July. I had share options worth$CAD 50,000 issued in 2017 (share options agreement) As part of the retirement process the share options were bought back by the company and they paid me the gain (based on a published EBITDA multiple) in September 23. However they paid the funds and taxed them as earnings under PAYE. The payment took me over the higher rate band of income tax so the tax paid was more than was due on a capital gain. Is it legitimate for me to claim this was a capital gain on my SATR? And therefore reduce the tax paid? I would prefer to claim it a a gain and believe it to be so, as the share options carried risk and only gained in value due to company performance. However, if I claimed the amount as a capital gain and the tax office rejected it, I would have missed the chance to contribute some of the funds to a personal pension. If I stated the income as a capital gain on SATR (April 24) and the tax office rejected the claim for some reason, would I then be able to make a pension contribution for the 23/24 tax year after April 24? In effect a pension contribution for a previous year? Or would my only option be to carry forward unused tax relief to tax year 24/25 ? Thanks for your assistance.

Hi Dave P38,
Shares sold upon vesting are subject to income tax, as advised at EERSM20193 (Employment Related Securities Manual).  
If the disposal is not at the time of vesting, then the disposal may also be subject to capital gains tax.  You cannot claim the difference as a capital gain in your tax return.  legislation dictates that is is income and taxed as income.  If the income is not included in your P45, please include it on the box on the employment page for 'Tips and other payments not included on your P60'.
Thank you.