Any data available publicly can be used. Some databases might ask for you to request access and this is fine also.
Government and academic sites (available for UK and many other countries) provide a lot of public health data that may be useful. Data is usually analysed in multiple formats (sometimes by region or other demographics. You may be able to link this to other information to help answer a question. Access to the raw data used may not be possible if the analysis you are looking for isn’t available, so make sure you can find what you need early in your project planning.
Image from Local Health (Office for Health Improvement and Disparities).
Take a look at the Library guide to finding statistics for other sources of data which might be useful: