Found! Another case of a London double marriage
After several days of unsuccessful Google searches for mentions of London marriages by German Jews in the pre-WWI period, it finally occurred to me to look in the archives of the JewishGen discussion boards—or possibly place a query there myself.
This message from April 18, 2013 popped up with information about a 1906 "double-date" marriage. The poster specifically asks the same question that I have posed: "Does anyone know a reason why someone, living in Berlin ... would have left Berlin **just** to marry in London?"
He also has an idea about that. He notes that his relatives in the two marriages were not born in Germany and were marrying those who were born in Germany. He wonders if there was a law in Germany prohibiting "aliens" from marrying, and if this could be the reason for the London marriage.
That is interesting. In the 1908 and 1911 Ringel marriages, all of the parties involved were technically aliens, having been born in Austrian Galicia before coming to Berlin. Betty Ringel was born in Rzeszow. Rachel Ringel was borth in Nisko. Their spouses were both born in other Galician (Austrian) towns.
So maybe if there was a law prohibiting marriages by non-citizen Jews, it applied for either or both parties.
On the other hand, another JewishGen poster from England has a different idea. I posted a reply to the 2013 message and have not yet heard back from the original poster, but Eva Lawrence had this suggestion:
"I think that the most likely reason for people to travel to London from Berlin simply in order to marry, was that they wanted a civil, rather than a religious ceremony, and that this was not available in Prussia."
Could be. The Tami Beck tree on Ancestry says the 1908 Weinstock-Ringel wedding was held at United Synagogue in London, so that would tend to contradict Eva's theory.
I've just ordered a copy of the marriage certificate from the UK General Registry Office, so I should have that point confirmed in a month or so.