@harrrrrry Think of it this way. If you pay giffgaff from your bank, there is one transaction. Using PayPal means paying PayPal from your bank, an internal transfer within PayPal from you to giffgaff, followed by giffgaff transferring the money from PayPal to their bank. That is three times the number of transaction, three times the opportunity for fraud and three times the cost.
I agree with @harrrrrry I would certainly prefer not to give my bank details to giffgaff, that is not information I hand out readily. From personnel experience when something goes wrong with a bank transfer it is very difficult and time consuming to rectify. Had an issue with EE took 3 months and my issuing a formal letter of intent to take them to the small claims court before it was resolved.