Beyond the Sea - 7/10. Was really fun to see the actors in this particular episode (including the cameo by the Caulkin brother who was great and appears to be a nod to Charles Manson as the Tate murders happened around that same time) and the general concept was smart and very Black Mirrory-y, but I struggled with some of the plot points (some of which have been discussed in this thread) and wasn't really able to to get past them. I'm also uncertain if the ending was entirely necessary nor what point was trying to be made (I base this statement on the tangential education I've received on mental health breaks in my profession and this seemed incongruent with the same).
This was a highly contrived episode even by Black Mirror standards -- there were a ton of technical details that were presented to the audience as "Look, we need this for the plot conceit to work; just roll with us, please." And then there's the stuff like how facial hair only grows when you're not asleep for some reason. But I liked the idea behind this episode and the bleak ending so much that I'm willing to overlook all the just-so stuff. I especially like the game theory considerations that the two astronauts were facing throughout the episode. It was a really interesting set-up.
Like you, I've only seen the first three, but I Loch Henry is my favorite so far. That one was really well-done all around. I anticipated
part of the twist (who didn't?) but the real twist is
how that videotape ends and oh man I did not see that coming at all.