How do I make sure that my store policies are correct?

Firstly, follow the advice that Lewis sets out in the 'Building Your MVS' cheatsheet. You can find the link to this cheatsheet within module 3.2 'Designing Your MVS'. Specifically, it is video 4 within the cheatsheet titled - 'Setting Up Required Pages'.

Using Lewis' advice you can make sure that your T&Cs and returns policies are all correct and in line with UK online selling.

You can start by using the Shopify template for your terms and conditions and return policy. 90% or more Shopify stores will use the templates provided by Shopify.

Also, you can check out your competitors and make sure that your returns policies are in line with theirs as they are already making sales and running their business with those policies in place.

It will be also become more clear when you sign your first suppliers as you can ask them to clarify their delivery and returns policies. They will tell you how they deal with returns / deliveries already. You can then replicate their deliveries and returns information onto your website.

If you want more guidance, you can check out the regulations that you will be adhering for the market that you are selling into.

In the UK, there are the distance selling regulations and also the 14-day cooling off period regulations as set out by the UK government. You can find information about these regulations here:
https://www.gov.uk/online-and-distance-selling-for-businesses
https://www.gov.uk/accepting-returns-and-giving-refunds

Another option for you if you would like to speak with someone about your business policies, you can also hire a lawyer on a service like UpWork. You can pay them by the hour and ask for advice on consumer rights, your policies etc.