The instructions on a situational judgement test may well request you select the item which comes to mind as quickly as possible and not to think about it. However, the choice of items to choose will be limited and the one you want to choose is not actually there. This creates a dilemma.
1. Understand what the organization wants, what is it's mission statement and what are it's values?
For example, if a house is burning down and the SJT is for the fire brigade an impulse might be to run into the burning building. Alternatively, the impulse might be to grab a fire hose first. However, the question may have left out information. Like saying whether there were actually known to be people in the building, or children. It is at this point any further decision you make could be on made up information in your own head. Going beyond the information provided could prove fatal to passing the question. So if the question indicates it is a residential building on fire, and the first moto of the Brigade is to save life which it lives by, running into the burning building could be the first thing you actually do. However, if there is not an option to choose, put on breathing apparatus first and take in an axe you will be flummoxed.
2. Contemplate whether you could apply for this post with a false application form.
If applying for a post online means a lot to you, then you want to try and get it right first time rather than be shortlisted out of it ASAP. So why not cheat? Create a false identity, email address and a real address you know exists. You could even go so far as to get a burner SIM card and number. If the first layer of the application allows you to do this, then you have gotten under their security. The employer cannot spend time vetting whether every applicant is genuine, the computer system for the SJT has to make an evaluation of whether this person has passed or not. Use a program to screenshot the questions and answers, or even take photographs of the screen questions and answers as they come up. If you fail you may be able to get feedback, and if the employer uses an automated feedback text program, this will give you an idea which questions you have failed on. Now you are armed with further information for another go. At your leisure, you can re-read through the questions and re-evaluate.
3. Spend time on the application process.
If you really want the job, then you are going to have to be smart. A lot of your spare time is going to be spent looking over the job specification, the job description, the values and code by which the organization runs itself. Or rather as one Occupational Psychologist called them the Espoused Theory of how the organization runs. For this let it be known what an organization says and how really does things is not the same. Some of those people don't follow the rules, they act differently and will have their quirks. They will be all grades of hierarchy. Some of those high level managers never went through this recruitment process and no doubt would fail if they did. The recruitment process is in many ways just a to wheedle out those who are going to be just as good at doing the job as anyone else but can't be arsed to think about what they are doing. Be different, you are going for a financial contract which is many thousands of pounds per year.
Getting a job is hard work, but you must commit to getting it. Otherwise you'll be stuck in a job or career below your capabilities. When opportunity comes be ready.