Are you actually in the UK posting to ebay UK from Auctiva?
Have you meet the minimum requirements, Feedback,Registered user for 14 days, PayPal, etc., to sell at Fixed Price? Here's ebay UK info on Limitations:
http://pages.ebay.co.uk/help/sell/fixed_price.html
If this is a new ebay account (no feedback) as stated, that might be the simple explanation.
Danno