Valuation on item

I'm not sure if your Order of Service has the provenance you're suggesting. You say your great-grandfather was a guest at the coronation - was that book handed to him when he arrived at Westminster Abbey for the ceremony?

A spot of research reveals that a company called Novello Co. Ltd published that book as a limited edition of 150. It's possible that as a publicity stunt, they gave a copy to Winston Churchill (then UK PM) and that's how it ended up as an exhibit in Chartwell. A copy came up for auction a few years ago with an estimate of £100-200 but it failed to sell. Which suggests to me that it was not given to a very small number of (elite) attendees on the day. Because if it was, each one that came up for sale would have the original owner's name stated as provenance.

https://live.adampartridge.co.uk/m/lot-details/index/catalog/9/lot/12476

Lot of publishers churned out hardback and softcover versions of the Order of Service which were sold to the public and snapped up as souvenirs of the day. I assume that's what you mean by the 'merchandise' version. They don't have a huge value as collectibles.....

The order of service book I see came in a numbered box which I dont have. It would have been given to him as he was a guest.

He might have claimed that when he came home to conceal an act of extravagance. My guess is that he bought it while he was in London.

During ceremonial events in the UK, an individual's importance is not recognised by giving goodies to some people and none to others, it's done by the strict implementation of what is known as the Order of Precedence. If your great-grandfather was a really important personage, he would have been given an arrival time later than most. The more important you were, the later you arrived. I cannot see any way how they could have handed out just 150 copies of that book when there was 8,251 guests in attendance.

Then he was probably given the book as a gift by someone in the royal family. An inscription inside, perhaps signed by the Queen Mother would have significantly enhanced it's value.