Added single thread performance is available with a Z chipset.
As of 6/19/17
What percent of samples can get an overclock
at a vcore around 1.4v.
I5-7600K
4.9 72%
5.0 52%
5.1 27%
5.2 16%
5.3 samples exist, unknown % of occurrence
What is your current motherboard?
What are the rest of your specs?
What kinds of games do you play?
Value for the money should not be the question, it should be
"what do I need to achieve my objectives?"
If budget is no issue, the 3800X or i9-9900KS is as good as it gets.
You will also need to change your motherboard.
What you see is pretty much what you get with ryzen as far as clock speed, boost, and overclocking.
OTOH, with the Z suffix intel 9th gen processors, you will get near 5.0 on all cores using the intel performance maximizer.
https://www.tomshardware.com/reviews/intel-cpu-auto-overclock-performance-maximizer,6179.html
With a £300 cpu budget, you are going to get a nice boost, regardless of the processor you pick.
You will also want a higher end motherboard like a X570 for ryzen or Z390 for intel.