Are you kidding me? 30 is genius. I went to Oxford for a Law summer school, and they said 22-28 is great. However, they do get people who get slightly less than that (and a Professor said that one person who was made an offer had a score of '9' in the recent years). This is all because they do not use the LNAT scores as a way of picking people, but combine it with all other aspects of your application. This means that if your LNAT score is below average (which I believe was 19/20 recently, but do not quote me on that), but your personal statement and GCSE grades are outstanding, then you should get an interview.
So in answer to your question, the average, and thus what is considered a 'good' score should be slightly higher, because there are more questions. But a score does not have to be 30 to be a 'good' one.
Hope I helped at least slightly.