is it for a round table, ie for the centre or are they having a long traditional type top table?
The main thing on a round table is people need to be able to see each other, so you either go low, or something very tall, you can buy huge martini type glasses (saw some in TK maxx) then the arrangement spills over the top and dangles down.
Long table tends to be low and hang over edge
Have a look at some florist websites for ideas, there is a lovely florists website with photos on www.tiger-rose.co.uk
You can do it a couple of days in advance and keep it somewhere really cool (easy at this time of year!) in a shed or garage. You would need to get your main flowers a few days before anyway to give them time to open a bit.
Use foliage from garden (you could even spray some of this silver). Silver, green and cream go really well together.
And little lights would really set it off.
What flowers are going to be on the other tables? Why is the top table display needing to be different? It would be best to try and have them linked together in some way.