Poinsettias are very picky and need a very careful handling. They come for Mexico so not exactly our Christmas weather. The 1st big rule is never buy from a supermarket. They just see them a seasonal cash maker, also never one that outside like some shops do. By the time they have moved all over the place and left on shelves. The damage will have been done long before you got it. When you do buy one always get it wrapped or cover it over. Before you take to the car or home. That short bit of cold is more than enough to do it in.
The hardest thing is to find a UK grown one , we only have one left the supermarkets have like so many others have bleed them dry.
They are expense to grow last time I checked with the trade anything under £7 meant selling at a loss.