There was some noise around Virgin and CityFibre recently, too - I think those will eventually end up combined, and most of the little altnets like Zzoomm will become part of CityFibre/Virgin or combine into a single non-CF altnet.
I'm old enough to remember Digital Region in the Sheffield area - it was an early FTTC network and went bust mainly because Openreach rolled out FTTC in the same areas and was easier to sign up to (I recall Digital Region was an absolute pain to get connected to, it took months). Their main asset wasn't a customer base (because they barely had one) but fibre in the ground, which got sold to (I think) Zayo for business and infrastructure use.
The same is true of these small altnets - Zzoomm is a minnow, its customer base won't be worth a great deal and its brand is worthless, but its physical fibre in the ground will at some point be valuable to a larger operator.