So can anyone explain why out of these three processors the 3 year old 6700K is the most expensive, and the price difference between the three models is only £35?
It really depends on how the CPU was made, it is possible there is a shortage of 6700k's and more 8700k changing the price. Also, it may have been more expensive for Intel to manufacture the 6700k vs the 8700k
Also, since AMD launched Ryzen Intel now has to compete some with AMD so they had to do something about there pricing with there new chips so they brought down the pricing on the CPU's that compete directly with Ryzen 3 5 and 7. So the 7700k and 8700k being most notable.
The 6700K is older, no longer being manufactured (AFAIK)... but it's still an extremely viable upgrade for someone with an i3 etc of the era.
It's been that way for years. Try finding a 4790K (new!) today! It's kind of a moot point, considering you could "upgrade" to the 7700K without replacing the board etc, but it's just the way the market for end of life CPUs go.... at least for a while.