Hey!
I haven't previously applied to S&C, so I'm not entirely sure on the specific formalities they expect. As they are asking you to upload a cover letter as a document rather than complete a text box on the application portal, I would personally be inclined to follow the usual cover letter conventions where possible, even if this is a slightly shortened version (i.e. the firms address on the left hand side).
However, given that you are limited to a single page, it may be that they don't expect (or even want) all of the traditional formatting if it comes at the expense of substantive content. If you are unsure, it could be worth sending a quick email to the graduate recruitment team to check. Alternatively, someone who has previously apply to S&C may be able to confirm what they did in previous cycles!
Hopefully someone with first-hand experience can share what worked for them - apologies I couldn't assist any further! Best of luck with the application