Difficult one to plan as the weather will be so influential over flowering times (as we all know from this year!!) & it might be a bit risky to rely on growing your own - but you can try and fall back on the GC if necessary.
What about some standard bays or yew or box topiaries (no flowers I know but they always look classy!) which you could put into large planters (black cubes?) and then buy stuff from the garden centre to underplant a few weeks before. Variegated ivies trailing down would look nice and at that time of year there would probably be palest primroses, perhaps some white anenomes and white narcissi available. The trees and ivies can be planted up well in advance - if you have somewhere sheltered to keep them so the leaves don't get scorched by wind or frost - and the underplanting can be done in the last couple of weeks before the big day depending on what is in flower / available.
If there is a colour theme to the wedding the underplanting could also contain a few splashes of that colour (primulas come in all sorts). Don't forget that garden centres usually have a huge range of colourful plants available in April (like sweeties - to tempt us!) and, even if you are buying tender annuals, it won't matter if you can keep them indoors the whole time.
Another plus - the planters (complete with plants) can then be moved to the garden and live a long and productive life.