Magicmike, that's just another link to the same provider. As the comment at the bottom of you link states, you can apply for finance by going to any school.
It's an unsecured loan with a maximum of £15,000. The whole point of the credit score is that you are assessed as a customer who will not default on payments. If you did stop paying for some reason you would find a debt collection agency chasing you reasonably quickly, and if you seriously couldn't afford to repay you may end up bankrupt, which will seriously affect whether you will ever get credit again.
As for improving your credit score, I'm not really sure what's the best policy. Certainly having a credit card helps, and never going over the limit or missing a payment. Whether it's best to put large balances on every month and then pay it off, or have a balance sitting there while you make minimum payments, I don't think is too important, but just that you use it responsibly and pay on time. Missed payments certainly show you as a risk for a large value unsecured loan.
Lastly I don't work for Tesco, it's pretty immaterial where you get your loan, but when I shopped around, with my credit score and my history, Tesco offered me the best rate. Doesn't mean that is the case for everyone, as loans are assessed on an individual basis, and the advert usually says 6% APR typical, doesn't mean you won't get a better or worse rate when you actually apply.
I'd go to money super market and get a quote there, then follow up the best providers.
The fact that they've marketed this as ATPL is absurd, in this context ATPL doesn't actually stand for anything, they're just another marketing branch of Secure Trust Bank plc trading as V12 Retail Finance.