If you recall in days of (very) old, ships accumulated barnacles/marine growth on their bare hulls so fast they had to be scraped regularly or they lost most of their speed. Then someone discovered that copper repels the little boarders and hulls started to be sheathed in copper. Then copper-bearing paint was invented and worked very well until the copper leached out of the paint (usually in a year or 2 for hulls always in salt water) and it had to be repainted. Great solution. Yeah, it was expensive stuff.
Then along came the tree (and barnacle) huggers, and the US EPA (and probably a similar agency in Britain), and banned copper paints, 'cause they hurt the environment. So now we have antifouling paints with some secret sauce (unknown to me) that (supposedly) works like copper but without harming the environment. And yeah, it probably costs even more.
Is any of it needed on model boats? No, because they are usually sailed in fresh water and only for short times.