Understand Your Credit Score and Actually Improve It

You checked your credit score and it said fair and you have no idea what that means or what to do about it. This explains how the UK agencies calculate it, the five factors with the most weight, three things people do that quietly tank it, and a six-month plan that starts with what you can do this week.

Explain how UK credit scores work and give me a practical plan to improve mine. Cover:
1. How credit scores are calculated in the UK: the three main agencies and what they each look at
2. The five factors that affect the score most, in order of impact
3. Three things people do that damage their score without knowing it
4. A six-month action plan: what to do in month one versus month six, and what takes time regardless of what you do
5. The credit products worth getting to build a history, and the ones to avoid

Assume I have a thin credit file or have made past mistakes. UK-specific throughout.