Either a small skipper or an Essex skipper, there isn't enough detail visible of the tips of the antennae to determine which one of those! Approximate location might give a clue as small skippers have wider distribution in the UK
There are also large skippers, but their wings are more mottled with dark brown, and therefore an easier ID.
(I had a discussion about skipper identification with an entomologist really quite recently - he said even people who can tell the difference up close sometimes record sightings of these as Smessex Skipper if they haven't been able to get a good look at the tips and underside of the antennae
There are also some differences on the underside of the wings but they don't often sit in the all the wings up position which doesn't help much in terms of ID)