On top of having a passport to go over the pond, tourists must have a pre-travel authorization, too.

On Jan. 8, the United Kingdom rolled out its new ETA system - and no, it does not stand for Estimated Time of Arrival. It stands for Electronic Travel Authorization.

It's a digital registration for U.S., Canadian and non-European passport holders who do not have a visa to enter the UK.

"It's £10, which is, depending on the exchange rate about $12 to $13, and it is valid for two years from the day you purchase it," explained AAA Northeast's trip advisor Wendy Marley.

Tourists must fill out a form found on the ETA app on the Apple store or Google Play; or online here.

Marley says it can take anywhere from three to 10 business day to process.

"So, purchase it for two years, you can travel for max six months at a time, you can go back and forth as many times between that time frame. If you do end up needing to renew your passport, you will need to get a new ETA," said Marley.

Remember to always make sure your passport is up to date and be aware of any other requirements necessary for the country you plan to visit.