A small handful of my students are UKSF. To pick up on a point raised by boropete, they all say that one of the hardest factors of selection is to persevere in isolation, the only thing keeping you going is the voice in your own head. Fairly soon into the initial phase the physical elements are largely equalized, because everyone is in a shit state, so mental strength supersedes athleticism.
By comparison, one of my colleagues in the US is former DEVGRU. He said that what got him through BUDS was his team, you just cling onto your buddies and tell each other that you're going to make it. However, to pick up on Scab's point, if we're talking about facing the Green Team for DEVGRU selection then he said that's a whole different story. His OTC lasted 8 months, and through that period his individual skillet was meticulously scrutinised for anything less than perfection. In his opinion, completely different ball game.
So, I don't really know what my point is, but it's in there somewhere. I've been up for 32 hours.