I can verify Gamseswat's story, at least the part concerning the sale of British games by US stores at bargain prices. The bulk of my collection came from two California dealers in the 90's. They purchased the cache from a collector who made a habit of going to a local department story during the Christmas season and buying their stock of allwins and the like. He then stored the games in a railway car that he had on a siding next to his factory. After his age progressed and his health deteriorated, he elected to sell the whole collection to the two dealers. I became involved when they listed the first game, Multiball by Handan-Ni. They sold me the machine at discount in return for my appraisal service for the remainder of their collection. I flew out to California for the transaction and during dinner, agreed to purchase their complete lot of 60 British games.
I can attest to the fact that many of these machines were retailed through American stores. How they got there? The only reasonable answer is the container trade from England that was rampant during the American lust for British antiques in the 70's. Who filled those containers? That's for you lads to discover. I believe that is not coincidental that the exodus of your coin operated history was around the time that bandits were legalized. Given the opportunity to win vast sums of money from the one-armed bandits, you abandoned your beloved allwins like a jilted lover. You must now win them back at greatly inflated cost.
As the saying goes, we don't know what we have until we lose it.
Lost in British historical gold in America