If Sophia (or one of her elder sisters) marries the Elector of Brandenburg instead of Hanover, then we've got at least an Anglo-Prussian union. Ditto if some plague carries off the future George II and his baby son, after his sister Sophia has married Frederick William I. Likewise again if GII's marriage is childless, like Charles II's, in which case Frederick the Great is next in line. Of course, all the above require Prussia to still become the dominant German power at some point.
Another one overlaps with the current thread about Princess Charlotte. If she gets to marry her Hohenzollern, that dynasty reigns in both Britain and Prussia. It's not a union yet, but if the marriage produces only a daughter, dear Papa may persuade Charlotte to marry her to Albrecht, youngest of FW3's four sons. That would seem safe enough at the time, as no one yet knows that FW4 will be childless, while his brothers Wilhelm and Karl will (OTL) have only one son each. Butterfly matters so that those two sons aren't born, or die unexpectedly, and you could be in business, esp if there's again only one child, so that if he's passed over Britain gets the unpopular Ernest of Cumberland. Finally, of course, if Wilhelm I lives as long as OTL, it will almost certainly be a case of the British King inheriting Prussia, rather than vice versa, which may, however illogically, be seen as less of a threat.