I'm a bit worried about the yellow flowered plant you have collected from a local burn, for two reasons. One is that if you bring in plants from the wild, then you will likely be seeding your pond with weeds such as duckweed, which will rapidly cover your pond in a mat of tiny leaves and is hell to try to get rid of. It's worth identifying the plant before you put it in your pond. If it's in flower now then it's not marsh marigold (which is non-invasive)
The other reason is that if this plant is covering a large area in the stream, think how quickly your pond will be filled with it.
Water lilies come in all sorts of sizes and suitable for various depths of pond, so they don't all cover the whole thing rapidly. They grow perfectly well in Scotland, and fish love the leaves to shelter under. For other plants, you will need oxygenators. These are sometimes floating plants, or sometimes they root into the mud, or some are in pots. Oxygenators are essential for a pond with fish. We have found this year that you also need an electric bubbler, which is far more efficient than a solar one. Fish produce a whole lot of poo, which sinks to the bottom and rots, so plants which can use these nutrients are essential. Buy your plants from a reputable supplier who is aware of which plants are invasive, and which can spread from a domestic pond into the environment, with sometimes disastrous results. It's well worth doing some research into ponds, so you get it right first time, rather than doing it on the cheap, with plants from the wild or from friends' ponds.