The Grooms are professional aviation Grooms who have gone through the same SEP etc training as the Loadmasters and the Gnd Eng personnel who also fly from time to time. In the UK it is the CAA, not the company, who decide who may fly - and they say a definite "No" on an aircraft that is not certified to carry passengers, as opposed to supplementary and support crews. Their issue is not the known family member versus unknown stranger aspect (though all the Grooms etc have gone through the same CRO screening as aircrew do), it is the need to self evacuate using, sometimes, special equipment that the evacuee has not been trained to use.
The difference between states comes down to the JAR get out that the interpretation of any JAR rule is a matter for the individual state - it's not that the UK are the top dogs or best at anythiong, just that my experience is that the UK will always take the most limiting view of any rule.