Guernsey has a very good and sensible policy on plastic bags. If you forget to take your own bag to the shops they will provide you with one for the handsome sum of five pence.
I try to be in the habit of unpacking my groceries and then taking the bags straight back out to the car so I always have some with me when I go to the shops.
Make sure you keep watching it past the initial 'bedroom' scene. Tim Minchin 'The Cure' for the plastic bag problem (am I showing my age with that joke?). Robert Smith watch out!