Currently I'm an LLM student in the UK. I wish to work as a lawyer in a top tier law firm in my country after my degree. Meanwhile I'm considering further PhD as it's my personal goal.
Any advice of whether it's worth spending 3-4 years and a large sum of money.

I cannot speak for the UK but in the US it does not, unless it is a Ph.D. in electrical engineering, Biochemistry, Physics, mechanical engineering, chemistry, bioengineering, etc., and you wish to practice patent law. In that case, a Ph.D. would be a very attractive notch on our resume. A Ph.D. in archaeology, English, political science, psychology, etc. (essentially, the humanities and social sciences), at least in the US, does not mean a whole lot to the firm.

Having a doctorate does not suffice in the legal world unless you are an academic.

Kind regards.