The British Crown, unlike its European counterparts, has been very particular about not allowing the use of foreign tiles without a Royal Licence to do so. In 1932 the King announced that no further Licences would be granted and His government wrote to existing title-holders asking them to forego the use of their titles.
See

https://www.heraldica.org/topics/britain/foreigntitles.htm#:~:text=In%20July%201930%2C%20the%20king,to%20be%20submitted%20to%20him.

This has not, of course, stopped foreign aristocrats and royals from using their titles socially, as the Radziwills had done. Like you, Carolyn, I think it very petty of the Queen/Palace not to invite the Radziwills under their Princely title.