This merge is the first sign of consolidation where Zzoomm has been sold to FullFibre.
If you actually look at the
https://fullfibre.co/isp-partners/ there's no mention of Zzoomm but all the other 14 different ISPs that are on the network.
This suggests that at the end of your contract you are free to switch to any of the other 14 ISPs on the network. The only thing I find strange is that why your speedtest would show as FullFibre if they are acting as a wholesale network!
I expect more of such mergers like this to happen to the other smaller Altnets who won't survive long term. However, it's interesting to see some of those wholesale ISPs are not much more expensive than what Zzoomm are offering! In fact, some are actually slightly cheaper.
Price factor will be the only concern when companies merge together. But as long as they are fairly similar then there's nothing to worry about.