If you are looking at the listing on eBay.co.uk, the price will be shown in pounds.
But if you look at the same listing on eBay.ca (the Canadian site) you will see the price in both pounds and loonies.
Be aware that the comparison is not going to be to the penny, since exchange rates change constantly, but it will give you an idea of what you will see on your credit card should you win.
Use the approximate foreign exchange rate indicated to bid in pounds. Paypal will take care of the rest.
Don't forget to allow for shipping and, if the item costs more than $20, sales tax, possibly duty, and the $10 Canada Post importation service charge, when you calculate your bid.