Merle - I don't post much on this thread - I mainly like reading!
I'm not so good with Boots fragrances. So many of them are quite heavy these days, and post-children, i can't do that. 
I think the poster recommending Chanel no 19 is on the right track. Perhaps you jsut need to spend some time sniffing lots of things to find what your next purchase will be?
Does it have to be purchased in Boots? If not, you have a lot more choice.
A lovely, widely available, Iris is "In Peace" by Space NK. Sold in Space NK! But if you already have the Prada, maybe you don't want more of the same?
I thought you might like the new Hermes scent: "Jour d'Hermes". It's very lovely, very light; a classy, feminine, floral. I think it might be just the sort of thing you like, actually. Also, hermes do a whole "Jardin" series of perfumes. I think you might like those as well. But my guess is that you'd like "Jour" more.
Problem is, that is not available from Boots.
The hugely popular modern feminine scents are Paul Smith "Rose"; stella "Stella" and marc Jacobs "Daisy". for a few years now, these have been the popular cultural interpretation of what a feminine, well-put-together fragrance is. Have you tried those? These are available in Boots.
Having said the above, I sense that their moment is passing. But it probably is worth smelling them, because whatever comes next will certainly have a connection to them. And having a sniff of these will help you define what you really like about the perfumes you really like.
By the way, I have found it really hard to reply to you because I am a scary Iris fan!! I've had to sit on my hands not to just post a list of my favourite Iris perfumes, and my favourite florals. i feel a real sense of affinity with you! 