Money: The U.K. as well as many European countries have their own currency. However, many establishments do accept U.S. dollars for their transactions. The rate of exchange is published daily, however if you are not familiar with its use, it is highly advisable to change your money to local currency at the Bureau De Change or a bank. Most hotels also provide the service of exchanging money but they usually charge a little higher rate for the service.
Cellphones: Many U.S. cellphones are manufactured to operate on a CDMA network that will not work in countries that use a different network frequency and type. Most European countries use GSM networks and although they appear to work the same as CDMA the two network types are very different. If you have a CDMS phone it is advisable to check with your carrier to see if they have international support because your cellphone most likely will not work in the U.K. If you have a GSM cellphone you will most likely have to purchase a SIM card in the U.K. and replace the one in your phone. In any event, it is always advisable to check with your carrier before leaving.
Passport: It is imperative that every U.S. citizen have a valid government issued passport to leave and enter the United States. A drivers license or other forms of identification are not acceptable and will inhibit your ability to board any flight departing the U.S. You need to make sure that you have a passport and that it is current for your trip. Passports can be obtained at any postoffice if needed.
Electrical Devices: If you plan to take any device that requires electricity to operate, you should use caution. European electricity is run at 220 volts and the U.S. standard is 110 volts. You will find that many hotels provide both options, but some do not. To be safe it is wise to check your device to insure it can run on 220 (some are automatically switchable), or another option is to carry a small power converter and adaptor which can usually be obtained at Walmart or any of the luggage stores. This small step may save your device from damage and avoid unnecessary aggravation.