With this type of investment, you are allowed to withdraw 5% of the original investment tax free in each policy year. You can use the unused party of a 5% allowance in a later year if you wish so if in year one you withdrew 2% for example, in year two you would be able to withdraw 8%.
Any withdrawal in excess of the accumulated 5% allowances, £1,250 per policy year in your case, will be treated as a chargeable event gain or an advance payment of your profit on the policy and that is subject to income tax.
If the policy is a UK policy the gain will be treated as basic rate tax paid and you will only pay tax at the higher rate of tax, 40% or 45% less the 20% basic rate tax paid, if the top-slice of the gain (the gain divided by the number of complete policy years) takes your income above the threshold at which you start to pay higher rate tax. The tax figure shown on the certificate is not what you have to pay to HMRC. That is the tax treated as paid already. Top-slicing relief can help to reduce a higher rate tax liability but not if the policy owner is already a higher rate taxpayer.
If the policy is an offshore policy, there will be no basic rate tax credit and you will pay tax if you are a basic rate or higher rate tax payer. You will still be entitled to top-slicing relief if the top-slice of the gain takes you into the high rate tax bands.
Take a look at HS320
for UK policies and at HS321
for offshore polcies. Look here
for some example calculations.
I hope this helps but le tme know if you have any further questions.