He made a pretty rambling, unfocused, and in my view incorrect argument in support of a divisive position, set against a well liked public figure. I think he's scored an own goal with this one.
Russel Brand describes science as a way to explain the unknowable. That's not what science is. He says science can never explain the 'why' of our existence. If he means why as in 'what is the mechanism by which we came to exist' then he's at least partly wrong. Science has given us lots of answers as to how we came about. If he means why as in 'what's the purpose of our existence' then I'd agree. That's not a question for scientific inquiry. He seems to mean the former though, because he says science can't explain the nature of consciousness or the universe. He should tell that to the neuroscientists and physicists working on just that, with some success.
He supports the cosmological argument for a creator. He accuses modern physics of requiring 'one free miracle' to explain the big bang, and then they'll take care of the rest. No. Physicists aren't asking for anything from religion. The cause of the big bang is unknown. It's okay not to know something. Ignorance is the default state until something is figured out. Not every unknown necessarily means 'god did it'. It no more requires a miracle to explain than lightning required Zeus. Even if we had never figured out electricity, "you cannot explain how this happens, therefore it is probably god" would not be a good argument.
He talks about consciousness in very mystical ways, and then gives the example of someone who had suffered a brain injury resulting in altered perception as being in touch with the global consciousness. As with the experiences of connectedness people have while taking certain drugs, I think a physical rather than supernatural explanation can be found here.
I think the weirdest thing he says is "No one can argue that when the Lion eats the gazelle it's not very nice for the gazelle. But what you can argue is that in infinite space that doesn't matter." (He then goes on a tangent to talk about the limitations of our senses, so I don't think I'm taking him out of context here.) I think he's bringing up this point in rebuttal to Stephen Fry's problem with suffering as an argument against god. This seems to go against his normal outlook and actions. If the gazelles suffering doesn't matter compared to infinity, our suffering is almost as insignificant, and everything he's crusading for is a waste of time. Of course the subjective experience of suffering matters.
Religion encompasses moral, cultural, philosophical, and supernatural aspects. I do not think there is any supernatural element to our universe. I can see why he likes the good fables in religious tradition, but think he will lose some credibility with atheistic types by buying into the supernatural.