That's awful of them. Do they know how difficult it is to go to the NHS for dental treatments??? I think your management should realize that having dental cover can really increase employee morale. Plus they would get urgent and immediate help when they need it.
I think all insurers have their own lists of dentists. But there are some insurers with more affiliated dentists, so you should choose them if only for the convenience.
The good thing is that most insurers no provide immediate cover for everybody in your organisation. We used to have problems with one insurer that insisted on having everybody checked first and then turned down some of our staff.