If you’re buying the phone outright then that could work. Provided it’s supported for that long, and you can look after it well enough then I don’t see why not - you just have to be prepared to pay £800 all at once.
Part of my reasoning is the £400 cost keeps my monthly bill down, as I don’t buy the phone outright.
If nothing else, keeping one phone for 6 years would be better for the environment.