Well, your pleco isn't a bn. They have very small dots & max out at 4 inches or so. At 6 or 7" he's probably not done growing...he's 1 of the "common" big plecs but I'm not sure & don't feel like searching planetcatfish for an id right now. It doesn't matter to his white areas. (sorry, a separate topic, lol). Big fish, big poop. Really your tank looks quite clean.
Are these new pics? I don't see much white splotches. Is that how it looks in real life now? I only see 1 rock & 2 pieces of wood. Does he try to hide in a very small space where he might get scraped? If he does you might move the rock or the wood away from the back/sides & each other so he can't do that. Plecos often like to have their eyes in the shade, try to make sure he has, if not a full body cave, some place(s) he stick his head when your lights are on.
I would just go with frequent large water changes for now. Every day if you can, as much as you can. I think you can't get antibiotics in UK shops. Let's think about your next step while you change water.