I help small and mid-size businesses and start-ups get the best out of Social Media and Digital Marketing.
I'll try to keep it short.
Try to browse through your close friends or LinkedIn people connected to you (1st, 2nd) and see who could be a soon-to-be-proposed or most-desired UK girl that is also famous.
Try to locate her agent and convince them to have the boyfriend select you as the "source" for the "rock".
Then have him and her mention your establishment in their interviews and social media (especially Instagram) venues.
Your only investment will be the price of the ring itself.
This would be a first step of course.