Post box - Royal Mail (Market Square)

This Royal Mail double post pillar box is located on Market Square on the east side of the old Market Hall. This is a very central position in Old Amersham, fine for pedestrian users but not so good for passing motorists as it's positioned between two roundabout junctions with double yellow lines on the box side of the road but a lay-by is provided on the other side of the road.
Double boxes are normally the main postbox in each town and usually located outside the town's main Post Office. Where is the Post Office in Old Amersham?
This box has been designated a NHS Priority Postbox during the COVID-19 virus pandemic per the sticker as seen in my photos.
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