It's simply a commemorative medal for Princess Victoria's 18th birthday. At the time (24th May), King William IV was still alive, though ill; he died a month later.
Her 18th birthday was of particular interest to royal watchers, because she could now inherit the throne in her own right. If she had been under 18 when William IV died, a Regent would have had to be appointed to rule in her place.
Things which indicate this coin was issued before she became Queen:
- her title as "heiress presumptive".
- her name, "Alexandrina Victoria".
Unfortunately, I don't have a British medal book to give you any further information on this particular piece.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis