The taxing of chargeable event gains on UK life policies is dealt with in HS320 here.
Who is liable to tax on the gain which is treated as basic rate tax paid in the case of a UK policy depends on who the insured is and who the beneficial owner of the policy is. If your mother was both the insured and the beneficial owner, then the gain on each policy will be part of her income for the final tax year of her life. ie 6 April to the date of her death. There will only be tax to pay if the top-slice of each gain takes her income for that final tax year in to the 40% tax band. The top-slice is calculated by dividing the gain by the number of relevant years which should be shown on the certificates.
If the insured person is not the beneficial owner, then the policy passes to the personal representatives of your mother and any gains are taxed on the estate. However, as an estate is never liable to income tax at more than 20% and gains will be treated as basic rate tax paid, there will be no tax to pay on a UK policy. Take a look here, here and here for more information.
I hope this helps but let me know if you have any further questions.