Don’t doubt core will run on an iMac, but what you would get with a NUC for a core is a low powered dedicated core running nothing but Roon, with attached external hdd for music, which can be anywhere in the house. It separates out the server & storage from the end point (the thing that actually produces the music you hear). The advantage being the end point can be a very small low powered device away from any major sources of noise or other signal degradation which tend to proliferate in computers running a Roon core which are multifunctional, especially desktop machines being used for other things at the same time.
End points can be be simple things like a raspberry pi or it can be built into to other high end devices such as amps or av receivers etc.
This is the perceived wisdom here on the forums and it also tends to be recommended by Roon themselves as the way to go. I too found it is the best solution for me and still do, just with different hardware now. Whether you notice a difference or not, depends on your equipment, your ears and the resolution of the music you listen to. The only way to find out for yourself is to try it and see. If it doesn’t show any worthwhile benefit, pass the NUC on.
Yes it does have a UK plug, It will need a 12-19v powers supply. I ran it happily from a 12V 5A linear power supply when using it as an end point.