I assume that it's not so rare with an English climate. Just get up early/stay up late every full or near full moon when there are risk of showers (virtually every day). The first and only time I saw a full arc I instinctively knew it would be possible five minutes before, and just searched for it. As said above, very beautiful in a 'it's so grey but there are colours as well' trippy sort of way! You got to see one once in your life at least, so brave those winter mornings...
I've seen one at night during a thunderstorm. It was around 3 a.m. and there was lightning striking near the Susquehanna River. The combination of lightning and rain made the rainbow.
In Hawai'i, on the Big Island, Heading out of Kona it is a common thing to see. I have seen a double arc, and It made my grandmother cry. But Night rainbows are nice. (As said 3 times now!)