Can normal seat belts be used with buckets?

[Slam Wagon]

Anybody know the law on bucket seats and harnesses, do you have to be using harnesses? Is it legal to be using harnesses and not a seat belt? Or are you ment to still be using the original belt even though its worse for you? Im running buckets harnesses and a harness bar, all declared to insurance so no problems there. But my harness bar mounts where my seat belts used to so they have been removed. Can be swapped back for mot so thats not a problem but just wondering about driving for the rest of the year.