Kimberly and Timothy,
Korean counterpart of Beetle/Kobee hydrofoil fleet is run by Miraejet Co.. English menu is not provided, but their on-line reservation procedure involves you being able to put proper information into a form in Korean, having a mobile number in Korea so that you can receive confirmation number by text, then wiring appropriate fare to the company's bank account in Korea no later than 3 days prior to the journey.
Booking by telephone or fax still needs to be followed by payment by the deadline, which would cost some hideous amount of fees/commissions if you attempt a cross-border transfer from your bank in the UK. (I've done the v.v. recently, and I'm not talking about the bid-ask spread in currency exchange.)
If you think you'll have an anxiety attack for not booking your ticket in advance, I might be able to help you through the first half of the booking procedure - typing a few characters and a couple of clicks, receiving the confirmation number, no big deal. But I will not be involved in the second, 'payment' part. That's what you need to take care of on your own, in time, once you're in Korea. You may e-mail me if you'd like.