Posted: 09/10/2014 at 10:08
It all depends on what plants. If you go to Spain then succulents tend to be cheap as plants like agave now grow wild and they pup easily.
The supermarkets tend be cross border so they have magnitude buying power, hence a plant might be 3 euro across their shops (say Carrefour). The UK supermarkets are only really found in the UK so they have less buying power. There is also the costs of bringing plants from Italy, Spain, Netherlands etc across the channel etc.
Rare plants though are different. They do tend to be much cheaper in the UK than in Europe and in the US.