By venturing into the world of oat-grasses I am entering difficult territory! In my view this is really the domain of experts and that is something I am not; not even close. The problem is that there are four oat-grass species commonly found in Southern England and whilst the pictures in the books show them as being different trying to distinguish them in the field, even with a book to refer to, is a tricky business.
False oat-grass (Arrhenatherum elatius) has "stems, sometimes swollen at the base. The spikelets are shining with two florets and a long, straight awn. Leaves with a blunt ligule" (Wild Flowers of Britain and Ireland by Marjorie Blamey, Richard Fitter and Alastair Fitter.) I rest my case, I frankly struggle with that even though I reckon I know what spikelets, florets, awns and ligules are.
Flowering from May through until September you can find false oat-grass on roadside verges, waste ground, meadows and other 'grassy' places. It can also colonise limestone cliffs and scree and so can be found along the Dorset coast. Also known as tall oat-grass it can grow to 2 metres tall.