I would say the most common BrE pronunciation is "You-choob". Technically, of course, it should be "t-yew-b" but even the single word is pronounced "choob" by a great number of native speakers. I note that Piscean said "commonly heard; not recommended". I agree with the first part but, for once, disagree with the second. You'll be perfectly understood (when talking about the website) whichever pronunciation you choose.