The plant is 'Himalayan Balsam' Impatiens glandulifera, it is an invasive plant found all over the UK now, especially near water. Go back and 'play' with the seed pods. they are great fun, stroke them and the ping open and explode, catapulting the seeds. Although non-native and quite invasive they are a pretty plant and the country is split between lovers and haters of it. The 'lovers' have aided it's spread through the UK by sewing seeds themselves because they'd like to see the plant in their area.