Quickly on this legal limit, the units of measurement may be different. The US limit of 10 ppm is measuring NO3-N, while our aquarium test kits use NO3. There is a difference of 4.4, so 10 ppm of NO3-N equates to approximately 44 ppm of NO3. I don't know what unit the UK uses for public water, but I would hope it is NO3.
Back to the problem...I have not had to deal with nitrate in my source water, so I will not guess at options but leave this for those who have the experience. There are ways to lower/eliminate these, and well worth considering. You did ask about Prime...this is (I think) the only conditioner that detoxifies nitrate, and Seachem have admitted they are not certain how this occurs, but it seems that the nitrate is somehow bound to become non-toxic. This is fine at water changes, as it immediately detoxifies the nitrate entering via the source water. However, Prime loses its effectiveness in about 24-36 hours, at which pint the bound nitrate will, if still present, revert back to the toxic form.
If the overall nitrate is 80 ppm, and 30 ppm is in the source tap water, that means an additional 50 ppm is occurring within the aquarium, and this you can deal with. Not overstocking, not overfeeding, regular water changes (these will still dilute the nitrate occurring within the tank), cleaning of the substrate and filters, and live plants. With respect to plants, the faster growing species are more useful as they take up more nutrients, and even floating plants which are fast growing and easier to manage would help.
Byron.