Whilst it is not a common butterfly, it is found at several sites scattered across a dozen or so counties, at at some of these sites it reasonably easy to see provided you know where to look and choose a nice warm day.

The Brown Hairstreak favours Ash trees and Hawthorn hedges, indeed they appear to roost and spend a large amount of their time in an "assembly tree" (which is often Ash) and often the easiest way to spot them is when they they descend to blackthorn below as the sun warms up the blackthorn.

Binoculars are often useful when butterfly watching and this is the case when looking for Brown Hairstreaks

Males tend to emerge a few weeks earlier than the females and usually tend to stay at the top of the tree until the females emerge.  Indeed the females are usually much easier to see and photograph when they descend to the blackthorn.

It's not common to see Brown Hairstreak in big numbers and I was pleased to see four together at Cowsden near Upton Snodsbury in Worcestershire in  August 2009.  I managed to get two of these four into a photo at the same time.
The picture below was taken the same day from a scaffolding viewing platform at a private site near Stock green also in Worcestershire where I was pleased to see and get a photograph of Purple Hairstreak as well.