I guess the real question there then is what are the rate differences for merchants? How much of a cut does PayPal take over credit cards?
Different companies/banks charge different amounts, it also varies depending on the amount of business they do.
The Paypal transaction fees are easily available on their site:
Purchase payments received (monthly) Fee per transaction
£0.00 GBP - £1,500.00 GBP 3.4% + £0.20 GBP
£1,500.01 GBP - £6,000.00 GBP 2.9% + £0.20 GBP
£6,000.01 GBP - £15,000.00 GBP 2.4% + £0.20 GBP
£15,000.01 GBP - £55,000.00 GBP 1.9% + £0.20 GBP
above £55,000.00 GBP* 1.4% + £0.20 GBP
I haven't dealt with card transactions for a while but when I did it was approx 1.8% + 20p per transaction apart from AMEX which was something silly like 2% + £1 per transaction (there's a reason most of the UK doesn't accept it).