Under UK residency - a plan on maturity is either subject to capital gains tax, or income tax, and your policy documents should advise which your fund will be subjected to, although it usually is capital gains tax.
HMRC would expect you to declare this, and if subjected to income tax, would be added to your annual income, and if capital gains, a charge would only arise if the gain was in excess of £10,600, but any tax suffered on maturity can be claimed to offset the UK tax position.
That's the normal way that this is treated, but a further consideration would be
1) As a non UK citizen, do you have the right to ask to be considered as liable to tax on the remittance basis or
2) Will you remit this money or part of it to the UK, if so then you have no option to be taxed under the arising basis.
But I will advise that to be taxed under the remittance basis will have drawbacks for you, as you have been a resident of the UK since 1998.
So you have been resident in the UK for more than 12 out of the last 14 years, so would not just risk losing your personal tax allowances but also come into being liable for the remittance basis charge, which will cost £50,000
Whilst I do not know how large the gain is within this policy, nor what your future intentions are with UK residency or whether this policy will be subject to capital gains or income tax, or even if you plan to bring this money into the UK, all of these are considerations, as advised above.
If you require further advise on the position, might I suggest you consult the policy documents or contact Old Mutual to establish the type of tax this matured fund will be subjected to, and then come back with that information, along with any other points that you feel might be of benefit/concern to you, based on the information provided.
If in the meantime you could rate the level of service, it would be appreciated, as this ensures I am paid, but you are free to come back to ask follow up questions on this matter.