Welcome to the community forum and thanks for posting!
I understand your concern about courtesy credit. In general, it is the amount refunded to you, if there are any corrections with the transactions for being charged extra, or other common reasons. To know the exact reason, please reach out to our PayPal credit team. Here's a link to the U.K. FAQ page showing that info. You should be able to find that contact number in the last questions. You can use the Courtesy credit at the time of checkout of purchases by selecting it.
If this post or any other was helpful, please enrich the community by giving kudos or accepting it as a solution.