All these questions are answered in the general section of the note 4 forum. I used to hate it when people would reply angrily to questions by telling the people asking the questions to search in a rude manner, but I can understand why they do it lol. I'm not telling you to search in a rude manner though lol but omg, the same questions are asked over and over.
The only eynos version that'll work on every UK 4g network is the N910C version. The snapdragon n910f is good but the exynos is better, better with overall snappiness, in games, and better sound quality too, I've had both versions
N910U is exynos cat6 while N910C is cat4. The U and C versions are somewhat international exynos. There's also korean only exynos versions, they end with the letters S, K and L, all these 3 versions are cat6 too.
Then there's a 3g only version n910H, Samsung couldn't have made this note 4 release any more confusing if they tried, cause there are even more versions out there! Hope you enjoy your notes