The £50 option looks the sweet spot to me where you take part in the second beta. The game should be fairly stable by then.
If I pledge £50 then can I increase it later today if I decide to play more or is it too late?
Remember the game will have expansions so its worth spending atleast £80 to get all of these for free.
Brabens vision for this game can be seen in the Dev Diarys, I suggest you go and check them out to see what he has planned.
If you dont go for £80 now you may actually end up paying more than this later on for the expansions when they come out, Like extra ships, Planetary landings etc etc.