If you are resident in the UK and receive income or have assets or carry on an activity outside the UK that would be subject to tax in the UK you may have to declare these in your tax return. The main exception to this is if a double taxation treaty exists between the UK and the other country. If there is a double taxation treaty, the terms of the treaty would define where the tax liability rests.
The disclosure facility allows you to report any tax liability from earlier tax years and receive a reduction in the standard penalty of 200% of the tax liability. The amount reduced would depend on if you came forward voluntarily and how much information you provided.
It is worth nothing that over 100 countries have committed to exchange information under the Organisation for Economic Co-operation and Development’s Common Reporting Standard. This is a very complex area of taxation and advice should be sought from a suitably qualified professional.