Hi,
SERPs, or Additional State Pension, is paid on top of your regular State Pension. Both are taxable income, if the total amount of the two exceeds your Personal Allowance, tax will be due on the excess.
You may be able to withdraw 25% of your total State Pension and Additonal State Pension tax free, for advice, p;ease contact the Pension Service:
Contact the Pension Service
You can see guidance on SERPs here:
Additional State Pension
Thank you.