If side-facing is your goal (ergonomically the best for a defender as that's what they were built for) then just get the proper side-facing seats- either a 2-man bench or the individual tip-up ones.
For side-facing seats, seat belts have never been a legal requirement. If you do fit them, lap belts only. Standard fitment was manually-adjusted ones, but retractable ones can be used too and are a bit tidier.
Legally it's a bit of a mess, due to the law not being thought up with old Land Rovers in mind. If they're of the age where they need a booster seat, they have to use it. But since no boosters are approved for use going sideways, they can't travel in a side-facing seat with a seat belt. But if there's no seat belts in the back, they can travel unrestrained. Which is a bit daft really since the side-facing seats are really intended for kids to use, and the lap belt can be adjusted properly to fit (and is surely better than no belt at all) so I don't see why a booster would even be needed.
Front-facing seats never fit particularly well in the back of a defender. They end up too close together and you usually have to fold one of them up to get in and out via the rear door. IMO this is dangerous in the aftermath of a crash or fire, and unacceptable as a solution. Rear-facing can be made to work with the seats up against the bulkhead, which gives good access via the rear door. But this does give minimal load space.