Hi angeloalbertini,
Unfortunately there is no specific or meaningful definition of an orthopedic mattress and a mattress that is a good match for one person’s back, body type, sleeping style, and personal preferences may be completely unsuitable for someone else to sleep on.
The goal of every mattress purchase is to choose a mattress that is a good match for you and that provides the primary support, secondary support, and pressure relief that keeps your spine and joints in good alignment in all your sleeping positions and a mattress that does this for one person may not do so for someone else … even if they have similar back issues. There is more about primary and secondary support and their relationship to pressure relief in post #4 here and in post #2 here.
As you will see in the links … a mattress that is too firm can provide poor support under some parts of the body just like a mattress that is too soft can provide poor support under other parts of the body. The goal is always to find a mattress that keeps your spine and joints in neutral alignment in all your sleeping positions and that provides “just enough” pressure relief in your most pressure prone sleeping position (usually side sleeping for most people).
While I’m not familiar with the UK market … the mattress shopping tutorial here has all the basic information, steps, and guidelines that can help you make the best possible choice and find a mattress that is a suitable “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and all the other criteria that are important to you no matter which country you live in because the steps would be the same in every country.
Outside of North America there may be a little more research involved to find the manufacturers and/or retailers that are knowledgeable, experienced, and transparent about the materials in their mattresses, but of course your comments or any questions you may have along the way that I can help with are always welcome 
Phoenix