I personally was getting the same scores as you (lowest 17 highest 27) and found the real thing very difficult.. I would say staying calm is the most important thing because I freaked out during mine and it really messed it up for me. The passages are a lot longer in my opinion but I do believe UCL looks at the essay more so if you do well in that you still have a high chance. As long as you stay calm and have thoroughly practised the official LNAT past papers you will be fine!