Physical Preparation: Make sure to buy warm clothes! I have lived in Hawaiʻi my whole life and I was so shocked at how cold it was when I landed, no matter how many times I was warned! Any winter item you can think of, gloves, scarves, big furry coats are all necessary here. One of the best things is that London is a really big high fashion city so you'll have no problem rocking your personal style.
Mental Preparation: In all honesty, there is not way to be 100% prepared for cultured shock. It was really helpful attending the study abroad meeting and learning about how the British culture differs from our culture. Don't let culture shock scare you too much though, it's one of the experiences that help you grow the most.
Visa: The easiest thing to do is stay in the UK for less than 6 months (which is more than enough time) so that you don't have to get a tier 4 Visa. All you need is your US passport and make sure you have the necessary documents for when you go through customs. The documents you need are your passport, the letter from the UH Study Abroad Office, the acceptance letter from the University of Roehampton, and a copy of your return ticket to the US.
Picking Your Flight: When choosing which flight I wanted to take, I decided to go through a travel agency so they could find the cheapest flight for me. The flight I got was about $400 USD cheaper than any other flights I could find and I didn't have to pay a fee because they ended up booking me with an airline they were partnered with. Regardless, flights to anywhere this far will be pricey so start searching early!