So in 1862 there was a greek referendum for a new king - Prince Alfred of Great Britain got 95% percent of the vote but still wasn't chosen, partly because of the London Conference of 1832 setting a precedent that Great Powers wanted to follow, but also because Queen Victoria was strongly opposed to sending a son off to a foreign country. So let's say she isn't quite so opposed (Prince Albert lives or something) and Alfred becomes King of Greece in 1862.
Now let's have a second POD of the Prince of Wales dying in 1863 (before conceiving Albert Victor, let's say), putting Alfred directly in line to the British throne. What exactly happens now? A personal union of the two countries really isn't going to fly - but all the alternatives are also awkward. Does Alfred abdicate the Greek throne after reigning for a year? Does he give up his claim to the British throne for his brother Arthur, Duke of Connaught? (Does Arthur somehow become King of Greece?)
Now let's have a second POD of the Prince of Wales dying in 1863 (before conceiving Albert Victor, let's say), putting Alfred directly in line to the British throne. What exactly happens now? A personal union of the two countries really isn't going to fly - but all the alternatives are also awkward. Does Alfred abdicate the Greek throne after reigning for a year? Does he give up his claim to the British throne for his brother Arthur, Duke of Connaught? (Does Arthur somehow become King of Greece?)