If you are the original policy holder (most likely), any tax liability will be to Income Tax.
If the surrender is a Chargeable Event. The Life Insurance Company should issue a Chargeable Event Certificate containing all the relevant details.
However, if the policy is a qualifying one, then the fact that it has been in force over 10 years, means it should be exempt from tax.
If there is an Income Tax liability, it is only on the profit of Â£1000, which is the excess of the SV (Surrender Value) over the premiums paid. The maximum rate is 20%, and only applies to HR taxpayers (or those who become so by adding the relevant gain to their income for the appropriate tax year. Top Slicing Relief may prevent/reduce any liability.
If in doubt, I suggest you consult your Endowment issuer to confirm that the surrender is exempt from tax.
I agree with Bob Fraser's points on retention, and looking at the TEP market, if appropriate.