They claim they can help you build your credit score
Able to get one willn't build your credit score but the usage will. The usage of a debit card/ credit card/ bank card helps building up a profile of yours with the tracking of how you use credit. Considering that citizens who have had a credit record, loose all of it when they are out of UK after a certain period(not exactly sure how long), that is why Britishers who have been living abroad for quite long run into troubles with credit institutions because of non existent records.
Did you read this part of their website where they mention how the credit record is built ?
Advanced Payment Solutions Ltd (APS) lends you a limited fixed amount which goes towards paying for all or part of a year’s use of the account, interest-free. The loan is paid to Advanced Payment Solutions Financial Limited (AFL), so no money changes hands and it’s never available to spend
Just repay the loan with 12 small monthly installments and we’ll pass on your payment history to the credit reference agencies
Make all your monthly payments on time and you’ll have shown that you can manage credit responsibly
Just seems like a way to play around the rules, but not illegal. So primarily they push your score up by showing you have used their credit responsibly.
And if your brother works he can get a no frills account from one of the high street banks, which is a more better option.