Provided you don't know that _all_ poppies are opium poppies, then it's legal to buy the seeds and grow the flowers. Of course now that you know
GP might not "know" that because it's false.
Only Papaver somniferum are opium poppies. The common red 'Flanders' poppy aka the Veteran's Day/Remembrance Day poppy (Papaver rhoeas) is not an opium poppy, nor are a number of others like the California poppy that are not even of the genus Papaver.