Your probly right,Its easy enough to put lap belts in.and i think in the event of a shunt it would keep you in your seat.So theres your answer, regardless of the law do whats best.I would fit belts as in a front end collision the baulkhead will act as a restraint but a rear end collision could have your passengers flying out their seats towards the rear of the van.
I'm am not one of those 'everything must be crash tested before I'm getting in it' types either, just reckon it would be worth while fitting some T:.