Go to your My Ebay, hover over the Account tab and select Seller Account. Near the bottom of the page it says "You can maek a one time payment using Paypal". Click that. You can then pay your seller fees that way, from your paypal balance. Even if you don't do anything, ebay will automatically take it from your paypal balance though.
I can't say for sure what happens in the UK if you don't pay it, but here in the US they would eventually suspend your account and likely would report you to a collections agency and they would harass you until you pay.