Comfort is very subjective. There are no universal standards of firmness in common usage in the UK. Because of this we don't give our beds a firmness rating like some manufacturers. Your main aim is to achieve good "posture" in bed. Too soft and your body will slouch - often leading to back pain - and too firm and you will cause discomfort at pressure points such as your hips and shoulder. Ideally the mattress should mould to the shape of your body while remaining supportive. When you lie on your side your spine should be horizontal and you should be able to turn easily.
Unfortunately the product you are looking at is exclusive to Mattressonline, so you cannot try it in store. The Bronze mattress is not categorised as an ortho mattress, so it is not one of the firmer ones in our range and is more likely to be consider a medium-firm mattress.
To give you some idea of the mattress feel you may like to try the Pocket 1000 Classic mattress available in Rest Assured high street stockists.