It is natural to be protective over your research data, especially if it has taken you a great deal of time and effort to collect/generate them. However, if a UK funding council funded your research, you are obliged to make your results and underlying evidence accessible to others.
Making your data findable online means that your research processes are transparent and your outputs are verifiable and replicable, thus demonstrating that your work is high-quality. Your data will be appropriately licensed in the repository, meaning that if anyone wants to use it in any way, they will need to give you appropriate credit for your work.
Open data also means that future researchers will not need to duplicate expensive or time-consuming studies you have already undertaken in order to advance knowledge. This means that researchers with fewer resources (for example, in developing countries) will be able to build upon your work without the need to obtain large amounts of funding or have access to specialist equipment.