In my view, the perfect commuter is a sportif/audax type bike, or light tourer. Almost as fast as an out-and-out roadie, more stable, with the facility to fit a rack so you can carry stuff without resorting to a rucksack - uncomfortable and unbalancing. And you can get something decent, and new, for less than £600; though obviously going secondhand might allow you to go upmarket.
GIANT have replaced the OCR and SCR ranges with the Defy. The entry level bike is under £500 and it certainly is not to be despised; I've seen reviews that rate it the best value on the market. You can see one here:
http://www.tredz.co.uk/.Giant-Defy-4...Bike_27728.htm
Similarly, the Dawes Sportif is a decent option at under £600.
http://www.dawescycles.com/p-335-sportif.aspx And if you decide to spend a bit more, moving up through that range to the sportif comp
http://www.dawescycles.com/p-336-sportif-comp.aspx gets you a really nice steel bike for £850.
Then theres the Kona ***** Tonk.
http://www.konaworld.com/bike.cfm?content=*****tonk Don't be put off by the silly name, this is a seriously good bike with an outstanding steel frame. At £770 (Winstanleys have one at £749, and you could probably find one cheaper on Wiggle or similar) it's a bit more than you want to spend but trust me, if you could stretch to the extra £150 you would not be disappointed, it's exceptional value.
Hope this helps. All of these bikes would make great commuters while being light enough, and fast enough, to offer serious fun at the weekend. And do think seriously about the Kona, a friend of mine has one and it is a very good bike for the money.