The iPhone 7 starts at £260 for the 32 GB version in Good condition, £330 for the 128 GB model and £370 for 256 GB. A pristine unit will set you back £425 (new units are available from Apple for £550), its available only in 32 GB flavor.
The OnePlus 3T is £170 with 64 GB storage, Gunmetal and Soft Gold colors are available.
The iPhone will get iOS 12, which was unveiled just a week ago. Meanwhile, OnePlus will provide software updates for the 3T until November this year and security updates until November 2019. Whether or not the phone sees Android 8.1 is unknown (were leaning towards no).