So it works something like this, you have the FEX you connected to, and that has connectivity going out of it to connect to ISPs.
There is CityFibre National, which includes the smaller players like AAISP, IDNet, Yayzi etc.
Then you have the likes of Zen, TalkTalk and Vodafone who utilise their own backhaul connectivity. I assume when Sky start selling CF next year, they will also use their own backhaul, so these become available if and when they connect themselves up directly.
Who is available first seems to vary from location to location, for me, I had no one at all for several months, the status was "we are done but waiting for ISPs", but then the first ISPs that appeared were Vodafone alongside all the CityFibre national ISPs. Zen took quite a while to appear, many months, and TalkTalk remains unavailable.
For me when ISPs were connected they appeared on CityFibre's own checker in the list.
There "which ISP" questions for some reason seem hugely popular on CityFibre.
I am personally on AAISP, but you have already ruled that out as you cost sensitive.
My general recommendation to people is one of AAISP, Aquiss, IDNet, and if all those are too expensive then go with Yayzi. Yayzi I would say are the best of the new guys, they did away with CGNAT a while ago, they are also active on social media, and even have a support forum.