Seat belts and the law

With the hot weather recently, it made me think about taking the hard-top roof off (if I ever get the bulkhead swap finished). It is currently fitted with three-point belts which are mounted to the seat tub and a reel which is attached to the hard-top van sides. If three-point belts are fitted, would it be illegal to use two-point ones when the roof (and therefore belt reel) was removed?

Thanks for any info.

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