I fitted lap belts to mine, similar age.
They're not mot tested, so no problem there.
You don't need type approval, and there is no issue with the insurance, as the number of seats are whatever the v5 states, or the van was made to.
(Mine is 2 on the v5, 6 on the conversion)
The forward facing seats needed seatbelts to comply with legislation. The rear facing I'm unclear on, but I never carry more kids than belts in the back. Strictly speaking even rear facing ones may if they have children in then.
I've also never been checked by anyone, home or abroad.
Very simple to fit, to comply with manufacturers guideline, so that issue is covered.
I used lap belt 210
http://www.securon.co.uk/seatbelts_layouts.htm#210
Securon sell 50mm by 42mm plates to use as mounts. They cannot provide information on how to use these mounts.
I drilled through the floor, and used (actually larger, spreader plates, about 75 x 75) to bolt the belts down.
Now, these comply with everything I could find that I need to.
You can always question whether they are safe, less safe than none, more safe, who knows without crash testing.
It's a thin tin box that'll fall apart in a major accident, the belts will help keep the kids in the seats in a minor shunt, maybe.
But it's legal.
(oh, the lap belt isn't approved for child seats, so ours travelled in the front when they where in seats, fortunately I only needed one at a time because of their ages/sizes)