If he has been added to the tank recently, fin splits would be natural. He has huge fins, and has been raised in a jar to keep them from moving so he'll sell. Out in the wide world, the fins move and rip along the edges. If the tail is fin rot you will see a greyish edge where it breaks off. You will have to watch him in case the tail fin starts to form solid pieces larger than the fin rays naturally there. They can get almost cones forming due to a fungus infection.
They can also just shed chunks of fin in a real aquarium where they can swim for the first time in their lives. When they lie around immobile in jars with those fins folded all over each other, things get trapped and fins get damaged. I had one of the many Bettas I kept whose tail fin shortened considerably after I got him. He lived for years.
I no longer buy long finned bettas because none f the breeding is for their health - it's all for our eyes.
If it calls for treatment, what is available is different in every country. You'd need a British betta keeper to comment.
