The main free resource is Bailii. I've sometimes also found cases on Casemine, which is free. The most popular paid resources are Lexis and Westlaw.
In general, the paid databases are more complete and provide additional useful features such as cross-referencing statutes and cited/citing cases, information on whether the case is still good law or not, and others. For this luxury you will pay a heavy price: typically a few thousand a year depending on what type of subscription you take out. If you are lucky you can get free access via a university course.
With that said, Bailii can sometimes have judgments not found on the others; particularly unreported judgments. Ideally as a professional you would have access to all three if you can afford to do so.
If you want a specific case and can't find it on any database you may be able to ask the court to provide a transcript, usually for a fee.