Okay, let's unpick this a little. There's two basic issues:-
-People's abilities vary, so individuals working at the same "training zone" level will go at different speeds, and
-People's view of what's "good winter training" varies - some people like to go harder, some gentler, some inbetween.
Basic solution to this is everyone rides on their own at their own pace (and on their preferred route - but that's another story
).
Next best solution is a range of groups going at different average speeds (assuming people in each group keep the effort consistent, and don't fall out over how to deal with hills and flats).
I agree that when you get up to groups of fifteen or more it's harder to make this work - you've got more varied needs/wants in that group, and you're spending more time following wheels. (Mind you, it shouldn't be impossible to agree on shorter turns, or just split the group while on the move. It just takes a polite chat in either case).
So, suggestion:- The Saturday mob gets split into groups no bigger than 10/12. These go off in the order fastest / middling / steadiest. People keep turns on the front to no more than a minute, and if they're suffering make them even shorter. (My usual line to people is that when you're on the front you want to be working at a level of effort you don't want to maintain for more than a couple of minutes max. A minute on the outside line, a minute on the inside line, and you'll want some shelter soon. This should mean the pace for those following wheels is still "endurance" rather than "recovery"). Everyone communicates so that if any change needs to happen, it's done with a bit of respect so no-one gets huffy and no-one spends twenty minutes between groups trying to chase on.
Does that sound okay? Should work for all concerned provided everyone accepts that a bit of compromise is required. Should also look a lot tidier - riders spread all over the Yorkshire countryside trying to chase each other just looks messy on what should be a group ride.