It is easier to file a form 12 rather than a form 11 so I'd go for the former. Just make sure you are de-registered from income tax for the year that you want to file a form 12 as you won't be able to file it if you are. You can make CGT returns on a paper form 12 but not sure about the electronic one. You can also file a CG1 if you want to file a form 12 (if you can't file an electronic CGT computation).
If you are in an espps scheme where you had to take care of the income tax element yourself, it's form 11 you file regardless of the amount of the gain. You complete the section for share options gains and input the amount of RTSO you paid. For tax years 2024 onwards, these schemes are taxed through payroll so there no need to report them for the tax year 2024 onwards as they are just included in your taxable pay.
Any subsequent gains you make on the disposals of the shares associated with the schemes after the income tax element have been sorted, fall under CGT, so it would be a CGT return (form CG1) you file.