Bank holidays are like Federal holidays where you can expect most offices closed except for shops and restaurants. Shops will be limited to Sunday hours. The British really like to take advantage of their long weekends, so you can expect heavy traffic on the Friday before a big weekend.
Unlike Federal holidays, they are not always days in order to celebrate something. They have many holidays that they do not give the day off such as St Patrick's Day, St Andrew's Day, St George's Day, Remembrance Day, Armed Forces day, and Guy Fawkes day.
Also if a bank holiday falls on the weekend, the bank holiday is moved to the Monday. In the US, when a federal holiday falls on a Saturday, it is usually observed on the preceding Friday. When the holiday falls on a Sunday, it is usually observed on the following Monday.
There are eight bank holidays (the US has 10 Federal holidays):
1 January New Year’s Day
Early May bank holiday (first Monday of May)
Spring bank holiday (last Monday of May)
Summer bank holiday (last Monday of August)
25 December Christmas Day
26 December Boxing Day
Occasionally, they will grant some days as bank holidays. For instance, the day of the Royal Wedding was a bank holiday.
People seem to get confused about bank holidays. It is worth mentioning that the BX and Commissionary don't follow UK bank holidays but they do follow US Federal holidays.
Also expect traffic to be busy like in the US on long weekends.
All the Best!
American to Britain