How is the fee calculated?
When purchasing from UK-based private sellers, the Buyer Protection fee is calculated as:
A flat fee of up to £0.75 per item, and
4% of the item price up to £300, and
2% of any portion of the item price from £300 to £4,000
There is a cap on the total fee amount you'll ever pay. Any portion of the item price over £4,000 will not incur any additional fee. When purchasing multiple quantities within a listing, the flat fee up to £0.75 is only charged once.
https://www.ebay.co.uk/help/buying/paying-items/buyer-protection-fee?id=5594