I dont know what the various ebay.uk fees are, but if you list on ebay.uk through auctiva, you'd get charged the same as if you listed directly on ebay.uk
the fees are the same whether you list through .uk or auctiva (that is to say, if you list on .uk through auctiva). the difference is that auctiva has its own scheduler, image hosting, and templates (all of which ebay charges extra for if you use ebay's versions of these)