Save is a relative term. A description of a standard of "safe" can be found online. (Note: Tritium does not penatrate the dead outer layer of our skin. However, the danger is in the injestion of the substance. Inhaled/ injested it equilizes and spreads evenly in the body. ) The standard is equal to or less than one dental xray. If the item, with tritium, is broken open within an enclosed area the exposure from it should not exceed that amount.
How much gasious tritium does the ring contain? Is the U.K. standard different than it is here in the states. If damaged what threat does it pose.
I would suspect the above may play a role in why or why not the item can be imported/ exported. Dependant on a particular country's laws.