I think you have hit the nail on the head there as that is the same conclusion I have come to - thanks to Mark Rowe over at 12stringbass.net - he has been helping me piece together the history...
Mark figured out why this bass showed up in London - it has been there since 1978. In the book about Hamer Guitars, "The Ultimate", on page 55 it reads, "Although the instrument (Quad bass) was intended to be built for Petersson alone, Paul Hamer mentioned the bass to Jethro Tull's new bass player, John Glasscock, who was interested in the idea. At one point when Tull was in town, Hamer happened to have Tom's bass in the shop for some minor adjustments, so he brought it down for Glasscock to see. Glasscock was so impressed that he ordered one immediately. Glasscock's Quadbass was completed just before the 1978 British Musical Instrument Trade Show in London, so Hamer brought it along with him when he flew overseas to attend the show. He asked Glasscock if he could display it at Hamer's booth and Glasscock agreed. Martin Barre arrived at Hamer's booth to demo some instruments and, while there, The Who bassist John Entwistle happened along. He examined the bass and, excited about the possibilities of the instrument, ordered one for himself in the Explorer shape of the (Hamer) Standard."
So yes - it does seem like this bass is that Trade show Bass and subsequently owned by John Glasscock of Jethro Tull! So we can assume that Entwistle would have played it at the show too! Mark also has a picture of legendary Nitebob holding my exact bass at that show in London 1978.
What a great story and an amazing bit of history. Thanks to everyone who helped piece this together!