This is an accepted solution.
I'm so glad to hear that you've managed to resolve the issue with your shipping rates, Taiki!
Now, let's move forward with your followup questions:
1. You can set up free shipping solely for UK customers by creating a shipping zone specifically for the UK. Once you have a shipping zone for that region, you'll be able to add a free shipping rate and you can choose whether to apply this rate to orders over a certain value, or on all orders within the UK.
2. To set a flat shipping rate for customers outside of the UK, you'll just need to create another shipping zone in Settings > Shipping. This time, you'll need to make sure that all other countries you ship to are in this shipping zone. Once the zone is created, you can create a flat shipping rate of £5 for orders between £0 and £49.99. Then, you'll create a free shipping rate just like you have done for the UK. This time, the minimum order value for free shipping will be £50 and you'll just want to ensure that there's no maximum.
Note: Since you're using shipping profiles, you'll need to ensure that you've added a shipping zone for each profile. I recommend referring to our help doc on shipping profiles for further explanation and guidance on this.
3. You can change your store currency in Settings > General. However, if you've already made your first sale, then this section will be locked. If this is the case, you'll just need to contact Shopify Support and they'll be able to assist with the currency change.
I hope this answers all of your questions! Let me know if you need a hand with anything else.