Plod will tell you yes, in truth it depends on your dads insurers, if they are happy to insure his use in an otherwise uninsured van then that's fine.
Been taken to court 3 times by norfolk plod/aoj for this, seen them get a bollocking everytime, even gwen wallace prosecuting for the aoj saw this was totally unjust, and she knows what she's doing as she's had me sent down before, but she is a really nice lady just hard in the court as she should be, get on very well with her .
tabetha
ps I won all three cases.