I'm struggling to find something that isn't a compromise is some aspect, at least something that'll work on o2.
Stumbled upon the Doogee F7 Pro today, which looks pretty good.
MTK6797 Helio X20 Deca core
4GB
5.7 Inch 2560*1440 pixels IPS, capacitive touch screen
32GB
Dual SIM
Support TF card up to 64GB extended
Android 6.0
4000mAh Battery
Dual Cameras, 13.0MP front camera , 21.0MP with Samsung 3M2 ISOCELL CMOS sensor back camera with flashlight and auto focus
http://www.doogeemobile.com/doogee-f7-pro.html
Should work on o2 and sugested price is $200. It's not available at the minute, but I've read online that it should be out June/July. Might end up getting one if the reviews are good and i've not bought anything in the meantime.
There is also a non Pro version with 3GB ram, 1080p screen and 3600mAh battery for $170