Nope, not cheaper. I think that scale comes into it. Some of the cheaper bikes use a smattering of mtb components to keep costs low, and they are really cheap. Fixies start at about 400 pounds or so, which is roughly the same pricepoint as respectable road bikes.
e.g. http://www.dawescycles.com/p-150-giro-300.aspx
http://www.dawescycles.com/p-170-mono.aspx
I'd give some thought to the Edinburgh road bike and the Giant Defy. Dawes, Raleigh, Claud Butler and Genesis usually have some decent entry level bikes that are designed for UK conditions.