Posters can always be put in house windows, and many shops may be willing to display a poster, particularly if there is local interest. Libraries often have an area set aside for local events, but it is always as well to get the posters to them in plenty of time, as from my experience they like to plan their displays in advance, and are also likely to have their own guidelines as to how long posters can remain on display.
In Southend, there are a couple of places set aside for general posting, but they always tend to be dominated by pop events, with each new poster at least partially obscuring those already up and generally looking very messy.