From money saving expert -
"If PayPal is used as a 'payment processor' by an online store, but you weren’t logged into a PayPal account, you ARE protected. A 'payment processor' is used in every card transaction to manage the secure transfer of data and funds from your credit card to the person you're buying from. Sometimes online stores use PayPal for this job."
https://www.moneysavingexpert.com/credit-cards/PayPal-Section75/#problem
As for going down this route, god knows but I'd presume PayPal are undercutting the payment processing fee that Worldpay were charging.