The mattress you refer to is not manufactured by Rest Assured in the UK, therefore, we are unable to comment on that specific product.
Key things to think about when you are buying are support, comfort and size. Most beds are available in firm, medium and soft ratings but these labels can vary from brand to brand. 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.
Remember that taking the time to try a new bed is a 10-minute investment to ensure you get many years of comfortable sleep. Most retailers will not exchange a bed if it proves not to be comfortable, so it’s very important to get it right first time.