These kinds of discussions have come up on here many times before. An argument for universalism is that it is efficient to administer and guarantees catching the maximum number of people in need. (There are other arguments as well, to do with such things as dignity and decency.)
Means-testing always misses a significant chunk of the people who are eligible for the benefit, for a variety of reasons, such as chaotic lifestyle meaning you don't have the wherewithal to claim, cognitive impairment meaning the same, or pride about not claiming benefits. This has been well-studied and is well-evidenced.
Universal benefits/credits etc of course also reach those who don't need them. But such people can and should be expected to pay for them through the tax system - if they are rich enough, paying not only for their allowance but for other people's allowances as well.
These same arguments apply to a wide range of benefits and also to things like university education. As an aside, I don't quite see why they don't apply to the NHS as well. Why don't the rich have to pay for doctor appointments? For reasons that I don't fully understand, this particular universalism is accepted while others meet with resistance.