US based so I don't know how closely this translates to the UK, but generally speaking there are three things that contribute to a strong credit score.
Length/volume of credit history.
This is a combination of how many accounts appear in your history along with how long they have been open. Having a series of accounts that were maintained in good standing looks better than only having one. Maintaining an account in good standing for a prolonged period (3+ years) is better than a bunch of short term items. "Ideally" your credit history should contain a mix of term loans that were paid per contract and a few (1?) revolving account that shows ongoing use. The goal is to show that you can handle ongoing obligations responsibly, and manage multiple things at the same time.
Or how much you currently owe vs how much people have agreed to lend you. Being close to your limits raises questions about whether or not you can really handle the additional debt. Having large availability raises questions about whether you would be able to handle it if you suddenly maxed things out. Finding the correct middle point can be challenging, the numbers I have seen thrown around most by the "experts" is 20-30% utilization.
Or how much new debt have you taken on? If someone is opening lots of new accounts it raises red flags. Shopping around for a deal on a auto loan or mortgage before settling on one is fine. Opening 5 new credit lines in the past 6 months, probably going to knock you down a bit. One of the concerns here is have you had the accounts long enough to demonstrate that you will be able to handle them in the long term.
One route that was suggested to me in my early years was to go take out a 6mo loan from a bank, and just place the money in a CD while I made the payments. Then repeat with a longer term. Worst case, you can cash out the CD to pay off the loan in an emergency, but otherwise it helps show the type of history they are looking for.
All that said, I have to agree with Pete B's answer. Don't play the credit game if you don't really need to. Or play it just enough to stay in the game and plan your finances to avoid relying on it. (Advice I wish I had taken long ago.)