I have little to no knowledge of the UK market but the process and the information that I usually suggest here in North America would work as well there as it would anywhere else. These “steps” and the information that will help you make better choices are listed in post #1 here.
The goal is to make sure you deal with retailers or manufacturers who will tell you every layer that is in your mattress.
If testing latex is your goal … then I would make sure that the mattress doesn’t have more than an inch (if anything at all) of polyfoam above the latex. Their spec sheet should tell you this (and if it doesn’t or they cant or won’t tell you what is in their mattresses then walk away.
Because of your condition … neutral alignment of the spine and joints would be more difficult than the norm so it would be especially important to make sure that the comfort layers were soft an thick enough to provide good pressure relief but not so thick that the heavier parts of your body were too far away from the support layers of your mattress that they sink down too far and put you out of good or neutral alignment. A mattress is always a balance between the conflicting needs of good pressure relief (which requires enough softness and thickness in the comfort layers) and good support/alignment (which requires a firm enough support layer and a comfort layer that isn’t too soft and thick).
You will see in the steps I linked that once you have some basic information about mattresses and materials and you have eliminated the worst choices (such as major brands, chain stores, and mainstream merchants) and know where to most effectively put your time and energy, the most important step is to “connect” with better local merchants and manufacturers (usually smaller independent manufacturers that are sold factory direct or through better sleep shops) that have the knowledge and experience to educate you about their mattresses and what is in them and give you the guidance that you will need to make good choices both in terms of suitability for your circumstances and in terms of quality and value (so you aren’t paying premium prices for inferior materials that won’t last)
The salesperson you were talking to is an example of the type of salespeople that I would avoid at all costs because they are more interested in promoting and selling what they have and that provides them with the highest possible profit margin or commission than they are in giving you accurate and helpful information and putting your interests above your own.
In North America … I have put together a list of factory direct manufacturers or retailers that are either good choices or at least better possibilities in most areas but I don’t really have enough knowledge or the ability to make phone calls there to do the same in the UK. You would need to do most of the research to find these in your area (and making sure they carried latex mattresses and could tell you the layers in their mattresses) before going to test mattresses. This article will give you a sense of how to tell the better merchants from the ones you want to avoid.
While it may be somewhat frustrating narrowing down the better options in your area and “interviewing them” before you visit them (finding the "experts) … it will save you a great deal of time effort and frustration and be well worth it in the end. Who you buy from can be just as important or even more important than what you buy.
As you come across questions or need to confirm the validity of the answers and information you will receive along the way … feel free to post them on the forum.
Being in the UK … you are at a bit of a disadvantage because of the research that has already been done here but step by step you will get closer to your own version of “the perfect mattress” if you focus more on who you are working with, before you spend too much time testing mattresses. Once you’ve done this then focusing on the quality of materials and on accurate personal testing with the help of the “experts” you’ve found will give you the best odds of success.
Hope this helps.