The original image colourised
This followed the end of the First World War. Families and communities were devasted, and they had nowhere to go to pay respects to their loved ones. Britain had taken the decision not to repatriate soldiers who died in the conflict and 300,000 soldiers were still missing and had no marked grave.
The wording has evolved over the years. The original quote from the Charles Dickens novel is simply, "Lord, keep my memory green". It's a plea from Redlaw (the Main Character) to God to not allow his memory to fade.
When this appears on gravestones as "Keep His Memory Forever Green" the meaning has changed subtly. Instead of referring to the buried person's own memory, it's referring to you (the reader's) memory of that person. So even when you are writing about more than one person, the reader has only one memory. This means that the script should read, "Keep Their Memory... " not "Memories".
The "Forever" or "Ever" was added as a poetic flourish and makes the statement seem more solemn. It sometimes gets merged with "Green" to create "Evergreen". This possibly accounts for the greenery around the arch.
The one in the Marston photo had to be built between 1917 and 1920. Once the main memorial was completed in 1920, the arch would be taken down. Two important points on the Marston evergreen arch photo, there is a First World War soldier sat at the front centre, and if you look to the right pillar (and some on the left) there are white squares, these are memory cards written out and left by loved ones to their fallen from the village, the main purpose of the arch, is a place for reflection.