How much?!? Just get a OnePlus 3T
Seriously, that's a truly stupid amount of money for a phone. A dual-SIM OnePlus 3T is £399 all-in for the 64GB model, £439 for the 128GB model, and their current production OS build is at Android 7.1.1 and is basically stock Android with no manufacturer cr*p to remove (*no* Bixby or anything like it), can be easily rooted if you want to go that way, and doesn't have a fingerprint sensor in a stupid place.
I know the S8 comes with a curvy screen but is it worth £400?...
</utterdisbelief?