Understand Your Credit Score and How to Improve It in the UK

Credit score advice in the UK is full of myths: that checking it damages it, that you need a credit card to build it, that a single missed payment is permanent. This explains what the score actually measures and who looks at it, the specific actions with the most meaningful effect over six to twelve months, the common myths worth discarding, and whether paying for monitoring is justified.

My credit score is [describe: never checked, recently checked and it was lower than expected, or you know it and want to understand why]. Help me understand: what a UK credit score actually measures and who uses it, the specific actions that have the biggest positive effect on it over six to twelve months, the myths about what affects it that I should stop believing, and whether it is worth paying for a credit monitoring service or whether free checks are sufficient.