The boardman hybrids are probably quite light. For that budget, probably looking around the 10-12kg mark. Avoid anything with suspension and look for a carbon fork
Halfords claim this at around 11kg, same for the women's
http://www.halfords.com/cycling/bikes/classic-bikes/boardman-hybrid-comp-bike
Disc brakes do add weight but they don't seem to have one without
Are you having to carry them upstairs hence the need for light?
It's quite hard to say as manufacturers often don't publish weights, so you'd be best off going to a few shops and asking / lifting some bikes