We have simplified the options into 5 categories, when it comes to mattress selection. Essentially the more dense the spring count within the mattress the firmer the feel. A 3000 pocket mattress will give you a good firm feel, (7/10), the density of springs offering great support and with many mattresses these days being double sprung in the central area that takes the most weight.
For a slightly less firm feel consider a mattress with at least 2000 pocket springs, although having the same basic specification, will offer a medium/firm feel (6/10). Moving down the firmness scale a 1000 pocket sprung mattress (5/10), will, due to the lower density of springs, give a softer spongier feel as you lie on it - some may consider this as the ideal mattress, but perhaps not ideal if you have any lower back problems.
There is also the option of a 1000 pocket sprung mattress with a 50mm layer of memory foam (5/10) - the ultimate in sumptuousness. For a very firm option perhaps consider an open coil mattress, which will give the firmest fell of all due to the stiffer springs and mattress makeup. Zip and link options available on all singles as well as non standard European sizes.