It's nice for a middle name to mean something to you I think. It's a chance to give your DD a name which you wouldn't dare use as a first name, but love. Or to honour someone or something. And it's good to have a middle name, I would worry that if I didn't give my DS middle names they'd feel left out, or like I couldn't be bothered.
I did a quick search on your posts (too much time on my hands tonight! DP working at a late gig and DS asleep) and I discovered you are a lefty (yay!) who's got a daughter with the same name as the new tory baby (it's still a nice name though!) you're Irish and like Prince.
So how about ...
OK then, firstly how about a name that sort of balances out the tory association of Florence?
Florence Bevan (After Nye Bevan - my DP knows a female Bevan)
Florence Mo (as in Mowlem, a personal favourite of mine 
)
Florence Emily or Florence Emmeline (after Emily Pankhurst)
Or perhaps simply a name that means red? According to this site at any rate, all these names mean red:
Florence Scarlett
Florence Fenella
Florence Delila (I thought that had "h" at the end?)
Florence Mackenzie (unisex, apparently)
Florence Cassie
Florence Merrie
Florence Rowena
Wow, there are a lot of these "red" names. I'm going to post this before my computer eats it, and come back to this ...