50ppm is the limit on nitrate in UK tap water. So you are at the top end of that. You could check your water parameters online, all the UK water companies should provide that. Their test results will be more reliable than your own. You have a home test kit, whereas they have professional tests available to them. I wouldnt expect it to be significantly different though. I test my tap water nitrate at 5 to 10, Severn Trent says its 7ppm.
Generally you want to keep nitrate at 40ppm at the top end. But, many fishkeepers keep fish at much higher levels than that just fine. Its up to you. If you arent seeing issues with fish then you could just live with it. Nitrate reducing media "may" help, although ive never used it and never really heard of anyone using it with noticable effects. If you try that it would be interesting to see results you get with it.
The plants you have tried are the ones i would suggest. You would need the plants under good quality lights to get the growth rates up and the nitrate down.
After that
RO or spring water would be the sensible next step. If you went down that route i would get your own
RO filter installed rather than purchasing it by the bucket.
You could try setting up your tank specifically to promote nitrifying bacteria which would take the nitrate and convert that to nitrogen gas. Its a technical solution, that would take some time and effort to produce results.
Here is a thread discussing the kind of set ups needed to produce this nitrifying effect.
https://www.aquariumadvice.com/forum...on-378784.html