Is it possible to promote my store to customers in Nigeria? Will they be able to make a purchase with their card? Will I receive payment in GBP?
Yes, as long as the card they use is a supported card type with Shopify payments.
If you want to set up Paypal payments, this could be better as Paypal may work with more card types then Shopify payments.
You will be paid whatever currency you set up in your settings