Wellsie, it depends to an extent what you want to do, what lenses you already have etc
I had a 550d for 3 years and loved it. I sold it recently to jump to full frame, i picked up a 2nd hand 5dMkII for £800. The detail on any pictures i managed to get in focus was a massive step up. That said, it was a learning curve. I actually used to get a lot more keepers with the 550 BUT when i got it right with the 5D the image would be superb (for me anyway!!). So it is quite a steep learning curve IMO.
The 5DMKII is an amazing camera but it never felt quite right for my circumstances, the majority of pics i take are of my daughter these days and i felt i had to put a bit more work into metering and exposure, tracking was sometimes tricky too. Im not knocking it but i did keep looking at the 6D reviews and thinking is it better ?
I assume your UK based as youve quoted £. Is there anywhere you can go to test them? Local camera shops are scarce now so i struggled. In the end i made the leap because the 2nd hand value of the 5D went up so i made a profit, and it meant for an extra £150 i could grab a 6D. All of the 6D Vs 5DMKII reviews helped too.
So far its early days but im made up with it. ISO performance was great moving from the 550d to the 5D and i can say its improved again on the 6D, its fantastic with ISO ramped up. Metering, exposure, colours seem a lot better. AF is more than adequate for me and does feel more responsive than the 5DII despite it not being much of a leap. The wireless functionality is a great little bonus and useful for including myself on some of the shots with my daughter. GPS i couldnt care less. I actually prefer the feel and weight of the 6D too. Yes its not as "solid" but with a flashgun, long lens and the 5D it felt a bit too heavy and clunky. My mate joked that he got whiplash of the shutter action of the 5D (he has a Nikon d600). The 6D has a nice shutter action and can be quitened if need be. In terms of controls, moving to the 6D will be more natural but i wouldnt worry to much, your instincts soon learn which button is where for any camera.
As the price was close to the 5DMKII i know for my circumstances i made the right choice. Weigh it up depending on your circumstances and what you want - if you do paid work you may want the 5DMKIII for the better AF, dual cards. It looks a fantastic camera. If you take mainly sports again it may be your better option. But you need to decide if the advantages it gives you are worth the extra money (£1k), for me they arent but for many they will be.
Trust me i spent ages going from thread to thread reading how great the 5D MKIII is and general 6D bashing but it depends on your reuirements. If you can get somewhere that has both and test.
Hope that helps a little!
Jonny