I think Turtledove missed an opportunity when he didn't have Britain ally with the Race.
They are a proper Empire after all, the Race should see them as one of the best of the great powers (Japan could be better, but Japan also mistreats Race prisoners fairly abominably).
It would require some work to set up, perhaps the Americans get pretty keen on decolonization of Britain's remaining territories, or start a trade dispute with Canada, or sell out too much for a German alliance, I don't know.
I just think it would have been an interesting move, and not totally implausible though it would have required some serious thought to figure out what sort of British government would make such an alliance, and how to get them.
You mean willingly, like Finland? Never actually though of that. An interesting idea.
I'm not entirely sure why Britain went pro-German rather than pro-American post war, since it makes so little sense. I see an Anglo-Race alliance as more likely, perhaps in return for concessions to Britain in its former colonies or something.