Hi!
For this kind of exercise (especially for a first-year scheme), I think it is unlikely that they will expect you to list lots of external news stories or have deep industry knowledge.
SWOT / PESTLE are just tools that you can use to help structure your thinking, rather than a tick-box exercise. What firms are usually more interested in is whether you can identify relevant commercial factors and explain their impact clearly and logically.
In most case studies, the key information will be in the material that they give to you. They'll often hint at political, economic or industry factors within the scenario itself, and they'll want you to identify them and analyse how those affect the business (for example, profitability, risk, reputation, strategy, etc.)
You can bring in wider knowledge if it is relevant and you are confident about it - for example, a clear industry trend that directly affects the type of company in the case study. However, this isn't necessary for these types of exercises, and it is better to analyse the facts that you've been given well than to force in external news without as much detail.
Therefore, I would focus on structuring your answer clearly (+ SWOT and PESTLE can help guide this), explaining why each factor matters, and linking everything back to the impact on the client / business.
I hope that assists!