I've no idea what plants may be available in the UK, but the best floater is Ceratopteris cornuta, Water Sprite, if you can get one. Once settled (it does not like changing environments/water sometimes, but just give it time) it will reproduce via adventitious plants from alternate fronds (it is a true fern so fronds not leaves technically). Other similar floaters are Water Lettuce (Pistia stratiotes) and Tropical (or Amazon) Frogbit (Limnobium laevigatum). For the latter, make sure it is the tropical species, as there are two temperate species that have basically identical leaves but do not do as well in tropical tanks, and in a warm discus tank would likely melt. The Water Sprite will thrive in high temps, another benefit.
Discus and cardinals will appreciate the cover, neither fish likes overhead light. And BTW, floating plants like most aquarium plants take up ammonia/ammonium as their preferred source of nitrogen. Floaters are especially good at this, which is why we tend to call them "ammonia sinks." Aquatic plants only turn to nitrate if ammonia/ammonium is not sufficient in balance for their purposes because they have to change the nitrate back into ammonium and this takes considerable additional energy that they prefer to use for growth or whatever, so it is a "last resort." Nitrates should remain very low because less ammonia/ammonium means lower bacteria/archaea levels so less (or no depending upon the pH) nitrite and thus less nitrate ending up. Nitrate should certainly never be above 20 ppm, and if below 10 ppm much better for all fish, esp these here. Some have nitrate at zero with good plant growth, ideal; my tanks for well over a decade have been consistently in the 0-5 ppm nitrate range.
I am going to stick my neck out here, and offer advice not directly asked for, concerning the substrate issue. Not everyone likes it when we do this, but I am conscientious and when I see pending trouble I like to offer help to avoid it.
The substrate is the most important aspect of the biological system in any aquarium. It is the prime filter bed for many species of bacteria/archaea that cannot live in the filter. Nitrifying species are not the issue, it is the multitude of aerobic and anaerobic species that provide a healthy biological system. Even a shallow (1 inch) substrate of fine sand will provide this, plants rooted in it or not doesn't matter. I can explain further if asked, but this really is an important issue.
