Most shops in Britain open at 9.00 am and close at 5.00 or 5.30 in the evening. Small shops usually close for an hour at lunch time. On one or two days a week - usually Thursday/Friday some large food shops stay open till about 8.00 pm for late night shopping.
Many shops are closed in the afternoon on one day a week. The day is usually Wednesday or Thursday and it is a different day in different towns.
Nearly all shops are closed on Sundays. Newspaper shops are open in the morning and sell sweets and cigarettes as well. In general, overseas visitors don't have much difficulty knowing where to buy things. Most shops sell the things that you would not expect them too.