This is the Gatekeeper Pyronia tithonus formerly and perhaps more appropriately known as the Hedge Brown or ‘Hedge Eye’. This butterfly can only be confused with a female Meadow Brown Maniola jurtina. However, in comparison it is much smaller and overall a much brighter orange. Where the Meadow Brown has one white spot in the centre of the black eye-spot on the upper wing, the Gatekeeper, as you can see, almost always has two. Also, with wings folded the Gatekeeper has a row of small white dots on its lower underwing, whereas the latter, if it has any, not all do, will have small dark dots. Plus, you are much more likely to find it like this, wings wide-open, basking on a leaf in a hedgerow soaking up the sun.
But beware, both species are subject to variation. I’ve seen Meadow Brown’s with two white spots and Gatekeeper’s with one. So it’s worth keeping a look out for aberrations. If you are lucky you may even find one of the heavily spotted multiocellata variations which can be very pretty indeed. Just remember to carry your camera, because of course I didn’t.