Don't know if it's unsuitable (will wait for someone more knowledgeable to discuss risks) but we've been doing the same: we started to see whether DS would take it, he did, so we've continued. I feel slightly better about it as we are usually making up a couple of bottles in advance (taking a risk I know, but doing so according to DoH guidelines), and so in doing so reducing chances of bacterial growth on bringing the bottle to room/body temperature.
It's fine - if they'll take it, save yourself a lot of bother with heating it up. However, it does need to be made up with very hot water and rapidly cooled and placed in the fridge straight away. Making up with boiled then chilled water from the fridge is not safe as baby formula can be contaminated with dangerous bacteria: it needs to be in contact with hot (70 deg c plus) water to kill the bacteria at some point.