Check your credit report (free) and fix errors
You have three main credit reference agencies in the UK — Experian, Equifax and TransUnion — and lenders may use any of them. Check all three (there are free ways to see each) and look for mistakes: wrong addresses, accounts that aren't yours, or a default that should have dropped off. Errors are common and drag your score down, so dispute anything wrong.
Two quick wins: make sure you're on the electoral roll at your current address, and close old accounts you no longer use.