I think saying the 1DX is worth £3000 more because the ergonomics are better is a bit strong.
You get a lot more with the 1DX in terms of features and the camera is designed for a different situation. You also get used to what you shoot too and in your case stepping from a pro camera to a 5D which is also a pro camera but on the higher scale of pro-sumer rather than pro of corse it will be different. The 1D is also designed specifically with the grip whereas the 5D isnt therfore there is only a finite amount of space on the back with the expectation of huge screens and the grip is an extra which makes things easier, but not fully integrated which makes the ergonomics harder to achieve.
But as it is a pro-sumer camera the size is important the 1D is too heavy and bulky to carry around for the more consumer side of the scale and thats whats nice about the 5D is its smaller size factor.
Its like chalk and cheese. Also depends on what your used to and how big your hands are. Kind of a silly argument really. The 5D is best of both worlds and for 99% of people not worth 50% extra, even for a lot of pros.