If it's on their own property/property tey rent it's not fly tipping, it may however be something for environmental health at the council to look into if it is actually encouraging rats (although unless there is food that seems unlikely in the short term).
At this time of year it's often a pain in the neck to get into your local tip even before any covid restrictions so unless they really are causing rats I'd give them some leeway for a few weeks, certainly I wouldn't go causing them trouble immediately just for a bit of rubbish.
Mind you I'm saying this knowing I've got a pile of things like old plant pots, concrete bags that have set solid, and crap wood in my back garden (not causing rats, and out of sight), that I've not been able to get rid of down the tip for a few months because of a combination of covid restrictions (booking a slot a week or two in advance is difficult if you're not sure you'll be around/able to make it within a 15 minute window and then get everything unloaded within 10 minutes), and the fact that the normal route to the tip is closed making it considerably harder to reach for me.
