Hi alexinedinburgh,
Unfortunately I have little knowledge about the Scottish market so I can only provide more generic advice.
There is more about the different ways that one mattress can “match” or “approximate” another one in post #9 here.
The most reliable way to know how closely two mattresses compare in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) will be based on your own experience and perceptions (which can be different from someone else because the “feel” of a mattress is very subjective). If you aren’t able to try a mattress in person before a purchase so you can know how they compare based on personal experience then you would be dependent on the information that was provided by an online retailer or manufacturer that is familiar with the mattress you are using as a “target” and how their own mattress would compare “on average” for most (but not necessarily all) people. Some manufacturers make or sell mattresses that are specifically designed to be a close approximation of some of the major brand mattresses such as Tempurpedic that are widely available. In this case I would also consider their return or exchange policy as a more important part of the “value” of your purchase and your personal value equation so you still have good options available after a purchase in case your own personal experience doesn’t agree with their descriptions.
In terms of the quality/durability of a mattress you can compare this based on the quality/density of the materials inside it. If you can find out the information in this article about each mattress you are comparing then you can make meaningful comparisons between them in terms of durability based on the guidelines in post #4 here. If you can post the information on the forum then I’d also be happy to make some comments about the quality/durability of the materials inside them. There is also more about the durability and useful life of a mattress relative to each person in post #4 here and the posts it links to.
Phoenix