Unaju in London
I would try one of the better izakayas, places like Kikuchi, Tomoe, Soho Japan, Akari, Asakusa. It probably features on some of the lunchtime fixed price menus, I think I've seen it on Tomoe's.
I've also seen it on the menu at Dinings as a special, there going under the name hitsumabushi which is the Nagoya version of unaju.
Out of interest when you were in Japan, did you eat it in an izakaya or a speciality eel shop?
first place i ever had this was at sushi yoshino on piccadilly place, a few years ago while still a wide eyed teen. It blew my mind. Have had it other places since (I'm a big fan of Kikuchi and always go there for sushi fixes now) but the grilled eel over rice at yoshino has endured in my heart. Good hunting, and do report back.