Have you heard of One Plus? I have their One Plus One, very happy with it overall for the price. Their latest model is One Plus 3T. I know someone that has it and they are very happy with it!
It is available sim free at:
https://oneplus.net/uk/3t else O2 do it on a contract if you prefer.
Another phone I came across that looks really good is the Axon 7.
https://www.debenhamsplus.com/p/1162...FRITGwodqOQGHw
However I don't know anyone using this but reading reviews it seems it is on par or even better than the One Plus.
EDIT: just saw the comments re: One Plus. Just to note the One Plus 3 has been discontinued. The new model is 3T which mainly has a better processor. This model is c. £400 but available on contract from O2.
To note re: iPhone, I have the SE (work phone), good phone but very small screen. You might want to go for the iPhone 6S which is same performance pretty much with a bigger screen. I got my wife one for Xmas. She is not "loving" it as you cannot do everything you could on an Android but I guess with an iPhone everything just works. Personally I would never move from Android to iOS anytime soon...