Once you have identified the datasets you think would be appropriate for your research, have conversations with data providers about your needs. Be specific about what linkages you require.
Having conversations with data providers early on may help to establish timelines for your project. This may be different depending on the datasets you hope to access and therefore time estimates on data provider webpages may not be relevant.
Has someone already curated this data?
The Health Data Research Innovation Gateway allows innovators to search and request access to existing UK Health-Related datasets including common linkages by disease type. https://www.healthdatagateway.org/
It’s best not to make assumptions about the data you will receive. Electronic health and care records are not curated for research as their primary purpose is for providing care to patients and service users. There may be variables missing in datasets that could cause issues for your project.
The national data opt-out was introduced in May 2018, following recommendations from the National Data Guardian. People can opt out of having their confidential patient information shared for planning and research. It applies to confidential patient information processed with section 251 support unless there is an exemption in place.
The Health Data Access Tool Kit (Health Data Access Tool Kit (hda-toolkit.org)) helps researchers in the UK understand how to access routinely collected NHS data. The toolkit signposts the approvals required and explores the issues that you need to consider and address within a successful application.