According To Gilbert and Sullivan, Iolanthe was a fairy who broke the fairy law and married a mortal, for that she was banished and spent twenty five years at the bottom of a "frog infested stream". However the fairies all missed her and petition the fairy queen who gives in and pardons her and at that point the opera begins satirising the English legal/government system.
So why Iolanthe? No frogs but a beautiful stream and waterfall which made me think fairies must live here.
Well done Bargee for first to find, thought it would be you, should have put money on it.
Highlights of a performance by The National Gilbert and Sullivan Opera Company can be seen here.
Note: If you look up the hill across the stream then you can - if the foliage isn't too dense - see a hidden tower that is part of Midford Castle.