This subject has always interested me over the years and - as 'boredintheairport' has mentioned - it's a good way to kill time on the journey.
I guess the answer is, it depends.
As already mentioned, if you are on one of the 'classic' sun routes to Majorca, the Costas or Orlando you can understandably expect near 100% UK nationals. However it has often surprised me how many foreign nationals are on other flights to/from Manchester. I remember waiting to board an EZY flight to Thessaloniki a couple of years back and listening to the languages and looking at passports, it was amazing to realize that this was not really a 'sun flight' as might be assumed, as there were not only plenty of Greeks but also Macedonians too (the FYROM kind).
Lots of Germans on LH flights etc. I would say a majority of Scandinavian flights have majority Scandies on board - Manchester's light seems to shine brightly in those parts.
Go further east and when traveling to eastern Europe destinations I have always been in the minority as a UK national. I would guess too that there are other routes (like the Iraqi flights) where there would be almost no UK citizens aboard.
My flights with Hainan have indicated around a 50:50 split (assumed) UK / Chinese in Business Class but it appears to be solidly Chinese down the back - although as always, it's difficult to tell which are Chinese nationals and which are British Chinese unless you see the passport..
What has surprised me most is the number of US citizens on our TATL flights even on TCX routes.