Hi. I have a different answer for your consideration.
Take a look at the HMRC helpsheet HS320 here. The narrative here and here backs up HS320.
A chargeable event gain is not subject to Capital Gains Tax when the policy is surrendered or comes to maturity or pays out on death. The only time that CGT might be payable is if a bond is sold to a third party who buys it as an investment.
If your mother was the insured and the beneficial owner of the bonds, then the chargeable event gains will be assessed on her estate for the tax year of her death as her death was the trigger event. Assuming she took out no more than 5% per annum, there will have been no previous chargeable events. In that case, the gain will be calculated by taking the value of the policies pre-death, adding the previous withdrawals and deducting from that the sum of the original investment and the previous chargeable event gains of which there appear to have been none.
The gains are then divided by the number of complete policy years they were in force and the "top-slice" added to your mother's income for the tax year of her death. There will only be an income tax liability if the top-slice of each gain takes her into the 40% tax band. The gains made from onshore bonds are treated as basic rate tax paid.
If your mother was not the insured or you were also insured under the policies and they were written in such a way as to pay out on a second death, you simply pick up the policies and their withdrawal/chargeable event history as if you had made the original investments yourself. The gains will be confirmed to you on chargeable event certificates issued by the insurance companies. The top-slices will be £6,667 (£60,000 / 9) and £3,333 (£30,000 / 9).
Unless your total income for the tax year of surrender including the top-slices is more than around £42,000, you will have no tax to pay.
You should read the notes in the third link I gave you above and take advice if you are unsure or let me know the precise circumstances as to how you came to be the beneficial owner of the bonds if that was the case as opposed to you surrendering them on behalf of your late mother's estate.
I hope this helps but let me know if you have any further questions.