To improve your credit score, you need to have some credit history. A credit card is a good place to start. I’ve just had a look at Loqbox. It is on the FCA register, and its parent companies are all on the register and owned by the same person. It seems legitimate. It also seems like a cheap way to build your credit score.
Since your money is held in Barclays, it is safe unless Barclays goes bust. If you want a cheap way to build your credit score, I would suggest starting with them for 12 months. If not, you could take a look at Barclaycard. Expensive, but it comes with perks and cashbacks.