Several Grapes have reported dissatisfaction with Royal Mail regarding lost, missing or damaged items. Their website gives advice on packaging, but I couldn't find any advice about posting plants, only flowers. There's an online complaint form, and they might even compensate you.
http://www.royalmail.com/portal/rm/j...diaId=15500193
http://www.royalmail.com/portal/rm/j...diaId=15500193