No they can be used when travelling it is legal to do so again its more of an advisory thing and you don't need seat belts but again it is advised that seat belts are not to be used if fitted according to the department of transport. Not something I would do mind
My brother in law got pulled up in his Sprinter van with a couple of his work lads sat on the groundwork tool box in the back and got a caution. I fitted a seat from a bus down at the scrapyard, it looked like an ex military vehicle and it had belts ready installed to the seat frame which made the job simple. The insurance were not bothered as long as they were notified and it passed MOT held in with 4 M8 bolts through 3mm spreader plates.
some insurance companies are refusing to insure vehicles with side facing seats such as landrovers as the injuries caused in a crash are far worse ie whiplash