Yellow Loosestrife sounds like it must be a relative of Purple Loosestrife. In fact there are two plants commonly known as Yellow Loosestrife (Lysimachia vulgaris and Lysimachia punctata) and they are of the Primulaceae family. Purple Loosestrife (Lythrum salicaria) belongs to the Lythraceae family.
 L. vulgaris is native to the UK whereas L. punctata is an introduction from south-east Europe and is sometimes referred to as Garden Loosestrife. They look very similar but the former has whorls of three flowers round the stem wheras the latter has four. Neither are as prolific and widespread as Purple Loosestrife.
 I saw the stand above in Edale five or six years ago. Actually the stems are so flowery I'm not sure which Yellow Loosestrife it is!