So here goes. Let's sort this out:
CoP is just efficiency / 100. There is no witchcraft. CoP needs to be treated in the same way as miles per gallon in your car.
I could claim I got 100 miles to the gallon this morning on my journey to work. I saw it on the screen with my own eyes. I would have got a photo, but that would have meant using my camera phone while driving and that's against the law. Admittedly, I was coasting down a hill. In reality, my car gives 30 miles to the gallon over a normal 1,000 mile stint. This is the only useful figure.
CoP is the same. All manufacturers publish a CoP at 7 deg C ambient and 35 deg C water temperature. This is the equivalent to coasting down a hill and is totally useless. It should NEVER be used.
What everyone really wants to know is what CoP will they actually get over a year's heating cycle? This is called Seasonal performance factor (SPF).
The MCS heat emitter guide gives some SPF figures. Unbelievably, these are actually very realistic.
In brief, they looks like this. If you are using underfloor heating, you will see a CoP of about 3. If you are running rads you will see about 2.7. If you run weather compensation, these figures will go up by 10 -20 per cent.
CoPs of 4 are a pipe dream. They are NOT achievable in the UK unless you run your radiators at 30 deg C or lower. Don't believe the hype.
Higher temperatures mean lower CoPs. You cant have high efficiency and high water temperatures; you don't get anything for nothing.