15% sounds impressive but it's 60 GBP in absolute savings. That might just be enough to finance the hours it probably took to collect, compile and process all things necessary to make this calculation.
However the UK has been running its power generation asset into the sand, we are increasingly reliant on things like https://ukpowerreserve.com/
So I expect that with the relentless push to smart metres (which is an optional EU directive, irony or ironies) we will see peak costs rise to battle the loss in cheap spare capacity.
if you look at their asset page:
https://ukpowerreserve.com/assets/
its basically small polluting gas an diesel gennies.
It took me half-an-hour to grab the CSV files from my monitoring, a write a scrap of Python to work out the difference in prices, then double-check my workings.
I'm happy with that ROI.