If you source and plant bedding begonias now in either pink or white, in a greenhouse, they would be outside and growing by the end of May and will flower all summer. You would be better to plant them in plastic pots the right size for your galvanised Because they have drainage holes watering would not be a problem. . The week of the wedding the pot and contents can then be placed in their gift pots.
I would not think 21st July too late to have flowering plants but you could choose either white or dusky pink lisianthus (cut flowers) from the florist, planted in oasis in your galvanised pots. You could cover the top of the oasis with small leaved foliage. They last very well. You have a choice of single or double blooms. Its a shame it was not considered earlier because you can buy lisianthus seed from Nickys nursery. I do not think you could guarantee to have them flowering in time. Have a look on her website to see what is available should you choose to use cut flowers because the florist would need to know which variety you prefer.
http://www.nickys-nursery.co.uk/garden-shop/seeds/cut-flower/lisianthus.-echo-champagne-30-pellets

Hello PF you could go to bunches and get stems of dendoriums orchids and tie with a nice ribbon and put in wee bucket.

Or get white geranium plants so they have a lasting plant.

White lobelia for the garden you could get the plants from your GC and grow on.

White Cape Daisy or Gerberas.

I will keep thinking for you but you had better get you scates on if you want these in bloom on the table.

Good luck .

We would love to see a photo of what you decide to give.

It would help to know what size pots she is thinking of using. Scotkats suggestions are all good ones and if you decide on the orchids you might be able to source single cut orchids in tubes from a flower market if there is one near you. They last a long time. Its certainly a novel idea and should please your guests.

Thank you both very much! I've been in panic-mode for a few days but feel calmer after considering these options. I think it will have to be a belt-and -braces job where I try growing some of the plants you suggest but have a cut-flower plan as a back-up! I'm not sure yet of the pot size but they can't be very big if they are to stand by the place-setting. I'm now thinking of growing two or three types eg white geraniums, lobelia, begonias etc, then there's a chance I can come up with the required number even if they vary in type. I have taken on board the pot within pot idea. I have to say I think cut flower arrangements would be much easier!
Thank you once again--I'll get going tomorrow!

Ikea do a galvanised pot at 89p each which is 10cm high by 10cm in diameter. Approx 4" each way. Because of the logistics in getting them to the venue it might be best to go for the beding begonia one to a pot or the lobelia (or allysum might be easier to find in white), three plants per pot. Try and get trays suitable for your chosen pot size at the garden centre or in the likes of Morrisons, they are usually quite happy for you to carry them off. It will make it easier to get them to the venue.
http://www.easyfloristsupplies.co.uk/galvanised/galvanised-bucket-7-5cm/p1352.html do a 7.5cm bucket.
You will probably have a florist supplier near you it might be worth dropping in to see what they have.

If you are growing flowers starting now, consider using Fast Flowers, the seeds are specially treated to make them grow within a certain timescale. You should find them in your local Garden centre. It will tell you on the packet what time is needed from planting to flowering. The packet will also have size of plant Ht and Spr. which will help you choose a suitable flower for the wedding.

Instead of putting the pots beside the ladies place settings you could put them in groups in the centre of the tables to serve as the table decorations. At the end of the meal either the bride or one of her attendants could tell the guests they are to receive a pot which will be bagged up ready for them to take with them at the end of the proceedings. Or alternatively get the flower girl to go round and hand out bags so they can bag them up themselves. Little bags like those used at childrens parties as 'goody bags ' would be ideal.

Wow Scotsgran! You are a veritable mine of information! Thank you for all these wonderful ideas I will certainly look for Fast Flowers and I'm emailing Sarah about the Ikea pots. I like the idea of grouping the plants in the centre of the table and will put it forward as a suggestion.
Thank you so much. I feel less stressed and more positive about it all now!

I feel for you Pennyfarthing, I have been through 2 weddings where the bride thought I was capable of using my magic wand to produce the desired result. I grew loads of forget me nots because my daughter wanted them and apple blossom in her bouquet. The florist could not get them so I grew them then she changed her mind so they are now self seeding all round the garden.
My daughter in law asked me to make an arrangement for the top of their wedding cake using the exact match to her wedding dress which had a lovely red velvet skirt. Again I could not source fresh flowers but spotted some silk flowers in an arrangement in a GC. I asked if they had them in stock or if they could get them in for me. They were the last of the previous years stock and the manager had spent all morning making his lovely display but agreed to take it down and let me have them. It can be hard work being a mum/ mum in law but there is also a lot of pleasure in success. Forget the stress and prepare to enjoy their day with them.

Many thanks Scotsgran and Scotkat for all your help! As you say, I need to get my skates on so I'm off to the garden centre (a different one) to make enquiries and do some buying up of seeds and plug plants!