I did a lot of research a few weeks ago and concluded the best value/quality for money one is the Kidde KID10LLDCO.
It's more expensive but a) it lasts for 10 years, b) you don't need to change the batteries (the additional cost offsets paying for 10 years of batteries) and c) it's a good manufacturer so you're less likely to die.
(you can also get this one without a screen, but I'd rather be able to see how much I'm dieing)
I think the one we've got is the previous model of the first one (screen, same general layout, different button shape) and it's been good so far and the British Gas service guys were quite happy about it (as were the gas fitters who installed the current boiler).
One thing I would say is to make sure you note the date that the alarm was fitted, they should be replaced every X years and usually have it noted somewhere like the battery compartment, I wrote it on the side of ours so I can see at a glance, not just when the battery is changed.