W Emmett, Builder, Decorator certainly chose a prime position for his advertisement. It's just behind the platform at Hounslow Central, visible to trains entering and leaving the station as well as to waiting passengers. This wasn't the first sign on the site, either: there are traces of others visible.
Emmett's business doesn't seem to exist any longer, so won't be complaining that foliage has been allowed to grow and partially obscure the view. The leaves seem to be fairly recent: an image in the Ghost Signs archive, taken three years ago, shows the sign virtually unobscured.