What is this brown insect with a large wingspan?
It is a moth from the Sphingidae family (hawk moths). They are generally large, robust and often have lobed wings (like the one in your pictures). I'm not closely familiar with this group of insects, and not with species found in the UK, but the individual you observed is very similar to e.g. Laothoe populi (Poplar hawk moth), which is also found in the UK.


(Laothoe populi, picture from Wikipedia)