Hi Tardigrade,
Welcome to the Mattress Forum! 
I’m sorry I’m not as familiar with the UK market, as this forum focuses primarily upon North America. I did find many mattresses using coir on the inside, but I suspect the fewer options in the UK with coir bed rugs may have to do with the flammability regulations specific to the UK. I did find an upholstery shop that sells 1" thick coir here. You could also seek out a synthetic fibre in the UK such as this, which is used in marine applications. Another options would be to contact one of the many mattress manufactures in the UK using coir and ask if they offer separate components and if they will sell you a piece of the coir separately.
As far as picking out a mattress for you, that’s not something that I’m able to do via an online forum. While I can certainly help with “how” to choose, it’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
The best place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
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.
The Sleeping Duck website doesn’t list the density of their memory foam, nor do they list the thickness of each layer or the type of latex being used (they do say it is natural). It does use a pocketed spring unit and has the ability to be customized with a trial period and different comfort options, so that may be attractive to you as you don’t get out much. I would phone them directly to learn more about their componentry and suggestions for your particular situation. You weren’t specific which IKEA mattress in the UK in which you had an interest, so you’d again want to find out the details of what is inside of any model you were considering in order to make an informed decision. If you’re able to find out that information and post it here I’ll be happy to provide some commentary on it for you.
Phoenix