Campanula carpatica come in blue and white forms.

marguerites might be nice, and should be flowering at the right sort of time. How small are the small pots? I would say get some tiny lavenders but it would be quite hit and miss as to whether you'd miss their flowering season. But on the plus side, there are lots of rather nice French lavenders and they come in blue shades and white. You might spot a theme here, but creeping rosemary flowers blue, too. (I do like plants that are either useful, difficult to kill or both!)
Geraniums (not summer pelargoniums, but "proper" geraniums) also come in all shades of blue, pink and white. There are thousands of them, and they're pretty much impossible to kill off which might be nice for the recipients who are less green fingered. I brought a couple of bits home from my friend's house in Huddersfield where she had unceremoniously yanked them from the soil, wrapped in a bit of damp newspaper for 6 hours in a hot car, and they're doing fine. I'm assuming here that you want to give outdoor plants. For indoor, I'd go with African Violets as again, you could have a mix of blues and whites. Sorry this is a bit of an essay, hope it helps!

Im after a Ranunculus Acontifolius 'Flore Pleno' which when I Google it comes up with lots of places selling it for addition in wedding bouquets (lovely white almost miniature double rose blooms) but very few selling the plants as such.

It may be worth a quick look as it seems to be a regular inclusion in wedding paraphanalia (sp?)

miniature white roses look nice--- whites easier to find than blue so you could look for blue pots or little blue bags like the florists do