Trade organisations
Trade organisations will often collect their data in an easy, usable format, for example, UK Finance.
Non-Official Data Sources
The main non-official data sources are those collected by university researchers. Some of these sites are huge. The most famous is perhaps the Penn World Tables, which provides data designed for cross-country comparisons of GDP and growth.
The ESRC, which funds research in the social sciences, has a data archive (at the University of Essex).
https://www.data-archive.ac.uk/
An American analogue is at
https://www.icpsr.umich.edu/
To access this last site you may need to go through the data archive at Essex.