On the positive side, if you have an unlocked smart phone, once you get here you can purchase prepaid SIM cards for both calls and data very inexpensively. They're sold everywhere - grocery stores, train stations, high street, malls. Just bring your phone with you so you buy the correct size.
On the negative side, do not expect internet connection to be anything like what you're used to from other countries. We won't have even crossed outside the border of Cambridge before our phones will lose connection with the internet. Have all your information for the day available offline, because you will not be able to count on accessing it wherever you go.
Related to above, have paper maps with you, because you can't count on having access to GPS through your phone. Many, many times we've been driving in the countryside and my husband needs to know what road to take while I'm telling him to hold on - the internet won't connect.
Everyone in the UK has a chip & pin debit/credit card. Most banks in the US don't issue them on a regular basis, but many will give you one if you request it. Do yourself a favor and get a chip & pin card before you come to the UK. The farther you get from big cities, the less likely it is that businesses will know what to do with, or even have the equipment to process, your US-style credit card. The other option, of course, is just carrying lots and lots of cash around, but I personally like to avoid cash when possible.
When I moved here I brought my flip-flops and sent my umbrella and raincoat on the slow boat, you know, because it was June. On another trip I packed light spring clothes because it was April. Both decisions were just silly. You can get any weather, any time of the year. Pack accordingly.
The tipping system is completely different here. Here's what we've figured out. Places where you prepay for your food, like pubs and cafes, people rarely tip. In restaurants, the standard tip is lower than the US, around 10-12 percent. But if you're paying for your meal with a credit card or your chip & pin card, know that the machines will rarely have that feature standard in the US where it asks you how much tip you'd like to add. You either need to have cash for your tip or ask your waiter to add the tip amount in ahead of time.
In the US, it's standard to be given your bill at the end of the meal. Here, it's considered rude to rush a customer by giving them the check unrequested. I've wondered how long one would have to sit in a restaurant politely waiting for the waiter to bring you your check if you didn't ever ask for it. When they start to sweep and put up the chairs? When the lights are being turned off? I've never had the patience and or nerve to test that out. Anyway, don't sit demurely waiting for your bill to arrive at the end of a meal. If you're ready to leave, request your bill (not the check).
On long road trips in the US, my kids will often ask for a treat at the rest area. Since they will have had nothing but fast food all day, I'll say something like, "Ok, but only if it's healthy." Ha! Generally the closest thing you can find to health food in a US rest stop is a bruised banana and maybe some Corn Nuts (corn is a vegetable, right?). Anyway, it was lovely to discover that England has Real Food at rest stops. There is usually an M&S or Little Waitrose (kind of like a small Trader Joes?) with things like a stocked fruit and vegetable section and fresh soups and sandwiches (not scary ones like you'd find at 7-11). OK, we still sometimes end up with something like a bag of chips and a Diet Coke, but it's refreshing that healthy is available.
If you are only traveling with adults, England is full of beautiful country inns and bed and breakfasts. But traveling with kids, I don't like this option as, one, I'd have to buy two rooms, and two, my kids are not impressed with quaint. They like wi-fi, t.v.s, modern bathrooms and a bed of their own. So we've found that Premier Inn is a great hotel chain here that has all the amenities my kids enjoy. The rooms come with a queen bed (although a UK queen is more like a US double) and two twin beds and the chain is moderately priced. I'm generally willing to give up charm to keep my kids happy on vacations. We've also had good luck with renting apartments through agencies like Airbnb, Holidaycottages.co.uk, and Skyecottage.co.uk.
Almost all of England is on the same school holiday schedule, so it's good to be aware of it. For instance, this year the kids don't get out of school until near the end of July. So if you were planning a summer trip to England in June or early July, you could find more availability and better prices. Also, any place remotely family friendly will be less crowded.
Depending on your trip, it can be worth it to purchase discount travel cards or memberships. If you're doing a lot of travel by train, for thirty pounds you can buy a Friends & Family Railcard, a Two Together Railcard, or a Senior Railcard, offering enough savings on train tickets to possibly pay for itself in one or two journeys. English Heritage is an organization that runs a number of historical sites. They offer a overseas visitor card for 50 pounds for a family. If you were visiting just Stonehenge and Dover Castle and bought the tickets individually, it would cost over 80 pounds.
Know that car rentals will almost always be manuals. If you're uncomfortable with stick shifts, particularly operating one with your left hand, you can specially request an automatic, but will pay considerably more. I've heard the recommendation, and think it's good advice, if you're flying into London, consider going without a car in London, travel to your next destination by train, and then rent a car from there. That way your introduction to driving on the left won't be in city traffic.
There's a lot of really good pubs, country inns, Indian food restaurants and fish and chip shops in England. However, they're not universally good, so doing a little research will improve your English eating experience. If you've tired of the above fare, and are looking for something safe and easy, these are the chain restaurants that we like in England. Nando's (Portuguese grilled chicken), Gourmet Burger Kitchen, Pret-a-Manger (breakfast foods, sandwiches, soups), Jamie Oliver/Jamie's (he owns a number of restaurants throughout England which we've had good luck at), Le Pain Quotidien (casual French), and Wildwood (pizza).