The Environment Agency have all their monitoring data available here: https://environment.data.gov.uk/water-quality/view/landing

They are endless .csv files which can seem daunting, but actually are fairly straightforward, these can be broken down by time-frame and region, and contain information on concentrations of all of the following…

Ammoniacal Nitrogen as N
Oxygen, Dissolved as O2
pH
Orthophosphate, reactive as P
Alkalinity to pH 4.5 as CaCO3
Nitrogen, Total Oxidised as N
Chloride
Solids, Suspended at 105 C
Nitrate as N
BOD : 5 Day ATU
Oxygen, Dissolved, % Saturation
Temperature of Water
Nitrite as N
Conductivity at 25 C
Ammonia un-ionised as N

That’s a lot of numbers, but as tempting as it might be to try to chew through them all, it will almost certainly prove more productive to focus on parts that seem important to your project and pare them down to something much more manageable and meaningful to answer your questions.

For Ecology and Fish data DEFRA have this really neat tool here: https://defra-ecology-prod.publishmydata.com/ecology-fish/

You can draw out an area on the map you want to know things about the ecology, it will pop up markers showing specific locations which have been sampled, with a key showing what was measured.

Fish stocks
Invertebrates (including BMWP scores which I hope you remember from first year!)
Diatoms
Macrophytes

This is a very new toy, so if you don’t find something in the immediate location, or you don’t feel the dataset is sufficiently complete there, don’t be afraid to pan out and cast your (kick) net wide or in places you’d otherwise struggle to go yourself for monitoring.

There are also guides on what the data is and how to use it here: https://defra-ecology-prod.publishmydata.com/ecology-fish/docs/