What System Meets My Many Needs?

If you are looking at handing over £10k I think that you owe it to yourself to try all the options before deciding.

There are currently only 3 FF DSLR players - Canon, Nikon & Sony.

Your current Nikon lenses aren't FF although the Nikon FF body will automatically switch to crop mode with them so they are usable - but other than your SB800 which I'm sure that you could trade or sell  I don't see your existing equipment as significantly tying you to Nikon FF.

However, after the recent demises of Jacobs & Jessops you are going to toil to find a Sony A99 to try in the Glasgow area. Even for Canon & Nikon FF Calumet in Oakbank Ind. Estate is probably now your only option.

& for something truly compact the  likes of a Sony RX100 may suit?