£40 iPhone battery replacement service? (UK Specific)

just wondering if anyone has used the optional £40 battery replacement service when buying a UK backmarket iPhone. How has the quality of the replaced battery been?

As Im ordering through there, battery life won't be the best which means I will have to get it replaced in a year or 2. Getting it done through backmarket doesn't seem too bad at £40 - is this battery replacement service worth paying for or not?