You should refer to HS320
If you surrender the policy for £55,000 having invested £20,000 18 years ago and you made no previous withdrawals, there will be a chargeable event gain of £35,000, £1,944 for each completed policy year.UK POLICY
If the policy is a UK policy, then the gain of £35,000 will be deemed to have suffered tax at the basic rate of tax of 20%, £7,000.
You will only have tax to pay on the gain if your income for the tax year during which you surrender the policy (let's assume 2014/15) including the "top-slice" of the gain, £1,944 in your case, exceeds £41,865. Assuming your pre-tax income for 2014/15 is £34,750, you will not have any higher rate tax to pay on the gain as if you add £1,944 to £34,750 you get £36,694 which is less than £41,865, the 40% tax threshold. As a UK policy gain is treated as basic rate tax paid, you have no further tax liability.OFFSHORE POLICY
If the policy is an offshore policy, then the gain of £35,000 will be deemed to have suffered no tax.
You will have tax to pay on the gain as no tax is treated as having been paid on an offshore policy gain. You add the top-slice of the gain to your income to determine your top tax rate. In your case, it will be 20%. So, your liability will be £6,998.40 (£1,944 x 20% x 18).
The calculations above assume that you are the beneficial owner of the policy and that it is not in joint names.PERSONAL PORTFOLIO BONDS
HS320 and HS321 mention personal portfolio bonds. I doubt very much that your policy is one of these. These produce a gain at the end of every policy year regardless of withdrawals and so they are not as widely held as the regular type of policy. If you had a PPB, you would have been sent a gain certificate at the end of each policy year and HMRC would have been notified also. There is an example of a gain calculation on a PPB here
I hope this helps but let me know if you have any further questions.