If you watched that video, his fish was covered all over in parasites when he looked up close with micoscope. Alot of people in the UK are just terrified of little bits of cold and they frequently heat pond when temps are still above freezing. Then if you look them up on UTube they are always so concerned about disease and parasites as they do not get enough long cold winters to kill them off. I seriously have not had a dead or sick koi in all my 27 years of running my ponds. Can't say the same about some of those fancy fat goldfish as they only seem to live about 10 years. I did have a old comet goldfish that lived to about 20 many years ago. Comets are heathier than fancy goldfish as they are not as inbred.
The point I would like to make, if you watched that whole video he said after checking his fish under scope, he come to the conclusion his whole pond was infested with parasites.
Meanwhile in MY Ice cold ponds with no heater for two weeks and night time temps down to -14 Celsius....