There are two main reasons for which top law schools ask candidates to write an essay. Firstly, they would like to check if a candidate can structure and argue a proper argument. Secondly, they want to know if a candidate has his/her own opinions and is not afraid to express them (which is really important in this area of study). For these reasons, LNAT essays have very various topics. Remember - you should not aim to show your knowledge on a particular topic, but to present a coherent and interesting argument. Moreover, essay topics are always controversial - it is done so for purpose. Author of a question wants you to engage in an argument and you would not eb able to do so if the answer was obvious.
You should leave the introduction until the end and firstly write the body. It is more convenient. You have a very little amount of time and it is possible you will not have much time to think well enough about everything you write. Sometimes ideas come while you are in the middle of the writing and it is possible you will not stick to your thesis statement. Therefore, it is better to firstly write the body and leave the introduction to the end.
The structure of every essay should be the same. Firstly you should introduce arguments for. So, in this case, you may write that not permitting abortion contradicts woman's freedom of choice. Also, it might be dangerous to her life in cases where pregnancy is a threat to woman's life. In the last sentance you should point out the values you base your argument on. So in this case, it would be freedom of every individual and the right to life.
In the next paragraph you should provide your reader with counter-arguments. You can point out that a fetus has a potential to become a child and at later stages of pregnancy it is very similar to a child. Therefore, there is no reason to treat it differently. If we punish people for killing a child, why should we not punish them for killing a fetus? Again, you should finish with values on which your argument stands. In this case, it is the right to life of a fetus (assuming a fetus should have right to life).
Third paragraph should bring balance to your argument. You should compare the arguments and deduce a compromise between conflicting values. In this case you have two steps to do. Firstly, you must deduce whether a fetus should be afforded the same amount of protection as every human being. One possible answer is that it depends on the stage of its development. A fetus in the 8th month of pregnancy is quite similar to a human being. On the other hand, a fetus in the 1st month is definitely not. Therefore, a line should be established that would mark the period from which a human being begins to exist. It is not a perfect solution, however is there a better one? Second step you must take is the balance of values. What is more important - freedom of choice or life of fetus? You should provide the answer with your own justification. While favouring one side of the argument, you must show that you appreciate the other one.
Finally, you should draw a conclusion, in which you provide a proposition of compromise solution that is based on your previous analysis. For example, you can claim that abortion should be allowed until certain month of the pregnancy. Again, you should show that you appreciate both sides of argument and justify your choice. In the end, you should write an introduction with a coherent thesis statement.
Remember - you have very little amount of time. You should allocate some amount of time to every activity - planning your essay, writing it and checking. You should strictly obey your internal rules.