I don't know the full context, but "So successful" appears to be emphasized, so it has been put at the beginning of the sentence. Normally, the sentence might say: "This Scottish one has been so successful" More context is needed to know what "this Scottish one" actually refers to, but the "it" in the second part of the sentence is referring to "this Scottish one." Using a more standard structure, you might say: "This Scottish one has been so successful, that it has been booked for the next two years." The meaning is saying that "this Scottish one" has reached a level of success and, as a result, has been booked for the next two years. "So" can simply mean "very," but in this case, it shows the extent to or the level up to which it is "successful." Does this make sense? If you need more clarification, please let me know. :D