When the hell will the final results be in? This long after a government-changing general election, the new PM would have moved into No. 10, and named their cabinet!
True - but this has always been a feature of the US system. Although a new president is elected in early November they never take office until January, the previous incumbent always remains president in that interim period.
And while I understand this seems mighty odd for Brits used to an election and a new PM the following day moving into 10 Downing St I can see some advantages of the US system. First it allows for a more measured and smooth transition phase (well it normally does, I'm not sure this is going to happen this time). But it also has some attractive features for the actual democratic process. In the UK if you need to vote by post you have to do that some days before election day as you vote cannot count if received later that the polls close, regardless of when you posted it. So it there is a major issue that emerges on the eve of a poll that won't be reflected in the postal vote. In the US (in many states, remember each state runs its own poll) as long as your postal ballot is post-marked on election day or before it will count (unless it takes crazy time to arrive), so even if you have a postal vote you can make your decision at the same time and with the same information as if you voted in person.
Now this necessary results in a longer counting process as you need to wait for all these mail in votes to arrive which can be days or even a couple of weeks after the poll. But the fact that the new president doesn't actually take office in January allows this.
