I wouldnt have lavenders especially if its hot then the marquee could be swarmed in beesi would maybe go for as sosas says roses or mixed basket plant arranged as surfinia, fuschia, bacopa, nepeta,etc and maybe do each table different?
We did something simular a few years back and we had cut birch trees tall in the centre and underplanted with mxd summer plants, it did look lovly with the standard margarites and 8-10 foot birch trees scattered around the marquee.