I have three suggestions:
Lockwood. He's trying to become inactive now, but I think he plans on coming back eventually. He's the most reliable ally I've ever had, and he's proven himself an excellent diplo player time and time again. https://www.warlight.net/Profile?p=2347081126
Simon. Simon, Lockwood and I have played a ton of games together and been allies on many occasions. One thing I like about him is that when he's losing, he doesn't just surrender and leave a bunch of neutral lands; he makes you earn his land by fighting for it, all the while trying to negotiate his way to safety, which is what diplos are all about. He's currently on vacation in South Korea, but he's from the UK. https://www.warlight.net/Profile?p=2455513373
Ordukai. His personality and playing style remind me a lot of Lockwood, only I haven't played with him as much. Regardless, from my experience, he's a reliable diplo player. He's also a good roleplayer. https://www.warlight.net/Profile?p=4581910313
By the way, including the profile links would help to check the bootrate as well.
Was just about to say that.