I can think of a couple that use some traffic free cycle trails in the Peak District but they are mostly on road...
"Knockerdown" 200km Starting from: Hulme End , nr Hartington
"Venetian Nights" 200km Starting from: Broken Cross , nr Macclesfield
I have decided to reduce drastically my road-riding. To this end I developed an Audax-style 200k using all the old rail lines from the above events. It has about 20 or so miles of road, in four short stages. I trialled this in August or September, when it was easily doable for me in 11.5 hours and on 23s and I'd been off the bike for 5 months following white-van induced surgery. It was a couple of k short, so I've fixed that and will sort out how to get a DIY by gps ratified and let you know about the route when I've seen if I've got it right. It should be pretty fast for most people, because, although the surfaces can be draggy and there are long uphill stretches, they are very gradual, because they were railways.
I doubt if I want it to be a calendar event because, while John's events are perfect in their use of railways, mine uses about 170k and a mass event might be completely anti-social and counter-productive under such circumstances.
C & A, if you are doing Dolgellau, I'll see you then!
Peter
PS I've also worked out two or three others, which use even less road, involving canal banks and Sustrans trails. These are much closer to the time limits because of the under-wheel conditions and gates and things - and my age, go on Pete, admit it! More later.