Someone's touchy, heh.
The fact you even mention a few hundred £'s, and decent in the same sentence for a Superking mattress, that was the trigger for the folk in here who were going to give you some advice, that can never be forgotten now.
Getting back to your original query, I ask this to a lot of people, spending 5 minutes in a mattress in a showroom will tell you nothing at all unless it is way, way off, e.g far too soft, or far too firm. You could dismiss a perfectly good mattress, or even worse think it the one you picked is fine.
Most good quality manufacturers/retailers will offer at least 50 day test period, and the like of John Ryan will offer a great deal more, this allows you to get used to the bed/mattress and ensure it is right for you, turning it, and checking for side sag, or bunching in the middle, and many other things that may become apparent, having a test across seasons is a good idea as well, to check how hot/cold it is if you are sensitive to that. You'll also find very few retailers and manufacturers are as transparent as John Ryan, they detail ever last material used, the gsm, how it is made, where it is made, and why they chose it.
Cost per nights sleep per person, 10 years/3650 days for two people 20p per night each is a great starting point for a King size, 10-15% more for Super. Cost per night goes up a lot when you end up replacing it after 3-4 years due to it being poor quality even though it may appear much cheaper at the start.
