Shops in the UK close much earlier than in the US. Some shops close at 6pm on weekdays and earlier on weekends, if they are open at all. In the US stores are typically open 10am-9pm and some grocery stores are open 24 hours a day.
Eggs come in a variety of sizes from 4 to 15 in a package. It is much more convenient than only being able to buy 6 or 12 at a time.
Within the boundaries of the greater London area is the original City of London, often called "the square mile" because that is the area it covers. The square mile has its own police force (City of London Police rather than Metropolitan Police Service) and an ambassador called the Lord Mayor of London. They also have a really cool crest with two dragons (pictured in bottom left corner of the ad below).
Monopoly at McDonalds has region specific names. Instead of Pennsylvania, North Carolina, and Pacific Avenues they have Regent, Oxford, and Bond Streets.
A4 sized paper is the standard used in the UK. A4 is about 8 1/4 x 11 3/4 inches whereas the standard measurement for paper in the US is 8 1/2 x 11 inches and is called "standard letter paper" or "8 1/2 by 11."
I recently sent a letter using a Freepost address. This is where a business pays for the postage instead of the sender. I've only seen this with business reply letters in the US but freepost seems to make a lot of sense. Instead of paying for business reply envelopes that may never be mailed back, you only pay for the letters that are actually sent.
In England and Wales you have 42 days (21 in Scotland) after the birth of your child to officially register the birth and therefore provide their name. In the US you are expected to complete the birth registration forms at the hospital and provide the baby's name then, sometimes after less than 24 hours. It can be more expensive and more of a hassle to wait to name your baby, which is why it is really common for American parents to have decided on the name in advance.
In the US 1-800 numbers are often provided to contact a company. These numbers are free to call. In the UK these calls are 0800 numbers and can cost up to 40p a minute. I had to make some calls last month and had an additional 40 pounds added to my bill!
Alcohol sales at grocery stores in Scotland are limited to 10am-10pm. I couldn't find any clear information about whether England has similar limitations but it sounds like it might be store specific in England.
This gesture (the v-sign with the back of the hand facing outwards) is offensive in the UK and is similar to giving the middle finger. I've heard (but there is no historical backing to this) that it originated during the Hundred Years' War. The French cut off the index and middle fingers of captured English and Welsh longbowmen so they were no longer threats. Uncaptured longbowmen would make this gesture as an act of defiance to their enemy.
This gesture is sometimes referred to as 'sticking two fingers up.'
