You won't go wrong with canon, nikon, fuji, panasonic or sony.
£100 should get you at least 8MP (usually good enough for A3) and probably face detection. However, not all sensors are equal and you may well find an 8MP camera produces sharper, less 'noisy' pictures than a 10MP camera. Also bare in mind that very few cameras come with memory cards these days.
Jessops' sub £100 range:
Jessops - Compact Cameras
Always check a couple of reviews from different sources before buying. Look for articles that mention:
Noise - how noisy the images are at different lighting conditions. A high ISO range is all well and good, but
image noise can ruin a perfectly composed image.
White balance - the ability of auto-white balance to choose the correct setting for the lighting conditions is important. As is the ability to override if necessary.
Optics - the quality of the lens in front of the camera is just as important as the sensor.
Auto-focus - the quicker the auto focus the better, esp if photographing kids. If photographing wildlife you need it to be as quiet as possible.
Shutter lag - the lag between pressing the button and the image being taken is important and can be the difference between a swan coming in to land and the said swan landed.
On the spec sheets look for:
Focal range - a focal range of 3m to infinity means that the camera is able to focus on images within that range distance. 3m is a bigger distance than most people realise, so want a focal distance of 1m to infinity at the very least.
Temperature and humidity tolerances - especially if you intend taking the camera abroad. As a side note always take your camera on as hand luggage. The hold gets cold quickly, allowing condensation to form inside electronic devices...