Hi EliFreeman,
Welcome to the Mattress Forum! 
I’m sorry to learn of your degenerative disc disease and your fibromyalgia, as well as your multiple issues attempting to find a mattress that meets your specific needs. 
An all-latex mattress is certainly quite capable of providing good deep support and alignment, providing the arrangement of the layers (density) is appropriate, and pocketed coil mattresses can also be quite good at pressure point relief, provided that the comfort material on top is adequate in amount, quality and plushness for your specific needs.
We only have 1 latex supplier in UK (LatexSense).
While this site focuses primarily on North America, I was able quite quickly to find multiple latex mattress fabricators and manufactures in the UK so you have other sources for latex mattresses besides Latex Sense.
There’s certainly no reason that this item, especially in their firm configuration (they offer soft and medium as well but are not clear as to what layers are changed to the different densities they list), wouldn’t provide enough support for a 120 lb individual. The key would be finding something that met with your particular needs and sleeping positions.
This item would also use good quality materials, as the Opulence, and it is available in soft, medium or firm, but overall it would probably tend to feel a bit less contouring than the Opulence at a similar comfort rating, as there is less material overall as compared to the Opulence.
The only way for you to determine this would be through your own personal testing. 10 cm of comfort material can be quite accommodating for many individuals.
Unfortunately, there is no specific configuration or type of mattress that is “good for fibromyalgia or DDD” in general because each person is unique, and a mattress that works well for one person with a specific condition such as fibromyalgia may be completely unsuitable for someone else with the same conditions to sleep on. In very general terms, softer and more pressure relieving materials that provide a more “relaxed” sleeping surface will tend to work better than firmer materials because for most people with fibromyalgia or DDD a softer more pressure relieving sleeping surface is a more important priority, but I know that this does vary with individuals, as well as varying day to day. Latex and wool have been popular combinations mentioned over time here on the forum, but again everyone is different.
A forum search on fibromyalgia (you can just click the link) will also bring up more comments and feedback from others that are in similar circumstances that may be helpful (but it will also confirm that there isn’t a single “best” combination of materials that will work for everyone with arthritis or fibromyalgia).
Nothing can replace your own personal testing, especially with specific health considerations. You’d obviously want to consider something that allows for enough surface comfort while sleeping upon your side and back, and also assists with pressure point relief, while still overall being resilient enough and provide enough support while sleeping. Overall, the two basic functions of a mattress are to support and to provide comfort (you can read more about that here if you like), with alignment being the first priority and then comfort coming second.
As you’re going through your readings, be sure to take a few minutes and browse through the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for, and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
In the end, especially with your very specific conditions, I wouldn’t draw on any else’s results, as they won’t be a reliable indicator of possible success for yourself. I would instead stick to looking at quality materials (latex certainly is one of them to consider) that are good at providing both support and pressure point relief, and I would focus upon becoming informed of any potential return/exchange policies should your mattress unfortunately not turn out as well as you had expected. I wouldn’t have as much of a concern regarding latex for support, as much of what you may be experiencing is the point elasticity of latex, which could be higher than what you are used to with the older coil mattresses you’ve previously tested.
Overall, the information on how to select a mattress, combined with the links to information about pressure relief and fibromyalgia, are the best “leaping off points” I can provide. I hope that information is helpful to you.
Phoenix