The lone McDonald and Giles album is quite good. Thanks for reminding me about it! I listened to my old vinyl copy a couple times today. It's perhaps not as consistenly good as the first King Crimson record, but it's more soulful, has a nice whimsical streak, and contains stronger jazz, pop, and folk elements than the mother group. I'd say in a way it has more in common with Caravan and the Canterbury sound.
I'd never thought about it before, but after listening to McDonald and Giles today, I can see how they could have had some impact on the early Enz ("Maybe" from Mental Notes comes to mind for some reason), especially the opening (and best) track on the album, the 11-minute plus "Suite in C," which has some beautiful (real!) string arrangements, awesome woodwind overdubs by McDonald, guest keyboards by Steve Winwood, and some excellent, quirky art-pop writing overall.
I don't know if it was ever re-issued, but track it down if you can and see what you think. It's one of those great lesser-known art-rock classics.
And check out Caravan as well. You won't be disappointed. Like the early Enz, their particular brand of prog/art rock seems much more timeless now than most (their early stuff anyway). They too mixed more concise, pop-song length tunes with a few pieces that featured extended instrumental passages. Like I mentioned earlier, I don't know if the Enz were aware of Caravan and the other Canterbury groups, but I strongly suspect they were an influence.