Korea - Japan Ferry

Hi, I'm going to Korea in April and as part of the trip would like to go over to Japan for a week or so before returning back to Korea. I know there are lots of ferries from Busan but I'm not sure of the best way to book from the UK, does anyone have any advice? I'll be in Korea a week beforehand so could book in Busan, but am worried that it will be too last minute. It would be nice to hear from someone who has some tips! Thanks