In civil claims, which is what CD falls under, the plaintiff, the person bringing the charges is usually the employee. In criminal cases this is the accused but we do bot have "the accused" in civil cases.
The defendent is usually the employer.
Additionally, "UK Law" is a misnomer, we have English-Welsh, Scottish and NI legal systems, ad of those three the Scottish is the most different from the other two.
Further additionally: Civil cases are expensive and you can't usually go to an ET if you've been with a company for fewer than two years.
The provisions under which a claim for UD can be made are found in the
Employment Rights Act 1996 s98 (2), which is well worth a read, informative and detailed.
Further also, if you're under probation or on a fixed term contract, all the information you've given there is moot as a company is by no means beholden to renew or extend your contract. They can also lay you off for any reason given if it's "due to the needs of the business". We have very few rights in truth, and are by no means "better" than the US legal system.
Edit: I'm not sure what your sources are. But it seems like you can't even correctly name the parties involved in a civil case. There's some very good information out there that acts as a starter guide to understanding various facets of law, like criminal, civil, tort, etc. If you'd like some pointers let me know. It's fascinating stuff.