Capital Gains tax is charged on gains in capital. Common sources of capital gains are shares and unit trusts bought cheap and sold expensive, property (both your own residence and other property), and companies that you may have built up over several years, (this also includes the capital gains from many share options or employee share schemes).

It does not matter where in the world the assets are, you are liable for the capital gains tax. (Ignore the blandishments of any company saying otherwise. The fact that an offshore trust service, etc. might not report a profit does not mean that it does not exist, and would mean, if you did not report it, that you would be evading tax. Tax evasion is a criminal offence).

Tax is actually levied on disposal of assets. This includes the sale of an asset, gift of an asset, and also the loss of an asset, (either because it is truly lost or destroyed, or otherwise becomes worthless).

Of these, the sale of an asset is the easiest to deal with - money is available to pay the tax on any gain. The loss of an asset is also easy - you end up with a capital loss that can be used to offset other gains. Gifting is something to be done with care if you will breach your annual exemption, as you clearly do not realise cash to pay any gain, but incur a gain nonetheless. In practice, relief's are available to deal with this problem, so seek advice.