I understand why they would allow older cars to stay on the road without safety mods, it would be unrealistic to expect that all cars get fitted with every new safety feature known to man...
That said, it would be silly to drive an old car all the time if it didn't have proper seat belts..
If you're only driving it on weekends, two or three times a month or even less? It's may not be a huge problem having lap belts..
My car is on a club permit, so I only drive it once a week at most. It could be 2 or even 3 weeks between drives and for the time being it only has lap belts in the front (none at all in the back), I want to get them upgraded, but I drive carefully to protect the car (and it's new paint-job) anyway, so for now I don't let it bother me...