I have a core i5 7400 and I am planning to upgrade. I have a budget of £450 and I have no preference of either amd or intel i am getting a new motherboard anyway.
On the Intel side, the I5 8400 is about the best you could do with your current budget, but the above Ryzen 2600x setup would be better anyway. The stronger Intel cpus go over your budget.
Sell your 7400 for some extra pocket money.