If 2024 is like recent years, the 16 will come out at the current price of the 15 this autumn. The 15 will go down by around £100 to the current price of the 14 and the 14 will go down to the current price of the 13.
I don't think you'll see much discount on those ahead of autumn. Maybe on the 14.
As the 13 will likely be discontinued in the autumn, we could see retailers trying to clear stock over the summer with decent discounts. And some networks discount further to encourage contract sign-ups over the quiet summer.
Reconditioned iPhones seems to be a big thing at the moment and you'll get better value with those if you don't mind it being reconditioned.
And the SE gets deep discounts as popularity of small phones continues to decrease.