I used to advocate against the use of combination alarms because the most effective position for a smoke alarm is typically on the ceiling, while carbon monoxide detectors are usually best mounted at about eye level on a wall.
However the growth in popularity of combination smoke and carbon monoxide alarms can't be ignored, so with my 'disclaimer' above I'll present some of the reasons this category is now growing so rapidly.
Combined alarms cost only slightly more than the individual detectors of either type with the result being that you save a lot of money having to buy only half the number of units. Installation is also much easier when you only have to mount half the number of alarms as you have to when installing separate smoke and CO alarms.
Smoke and Carbon Monoxide Detector Reviews
In my opinion the Kidde KID10SCO offers the best up-front value for money of all the currently available options.
This is the most expensive option currently available but it is the top seller on Amazon.co.uk because it has a range of features not yet available in other models.