Dallas here from the Social Care Team at Shopify.
Great question. When it comes to what your customers pay, however, that will be entirely up to what you decide.
With drop shipping, the transaction from your customer and the transaction for the product with oberlo are actually separate. In that case, you would charge your customer the price that you choose.
Say it costs 5 Pounds on Oberlo, and you want to include all that extra fees, taxes, and markup for profit, and you choose to charge your customers 20 Pounds that is what your customers will pay you, and that is what will end up in your payout. Then from there, you will go over to Oberlo where you will use your credit card to pay the 5 Pounds to send that product to your customers.
You will be using your credit card, but since you received money from your customers already, you are still making that profit.
Therefore you can charge your customers what you want to/need to charge them on your actual store. Does that make sense?
When you import a product to your store, you can add a mark up in Oberlo before you import to Shopify. That would be found on the Import List under Variants on the Oberlo app.
Alternatively, if you would prefer to adjust this on the Shopify side you can do so by going to Products > Click the Required product and then changing the price in there.
For any technical questions, however, into what you should be charging them for VAT should be directed to a local tax professional. They will have more direct information in regards to that aspect.
If any of this doesn't make sense or if you have any more questions, please let me know. I am happy to help!