With a bit of Googling I found this page which is dated February 28, 2022 (so not too old):
https://dgtlinfra.com/kkr-hyperoptic-uk-fiber-passings/
"Presently, Hyperoptic serves over 230k customers in the UK with its fiber-to-the-premises (FTTP) service, implying a ~30% penetration rate. Additionally, the company aims to maintain a penetration rate of at least 30% where its fiber network is over two years old."
So that gives you a good number to play with, 30% penetration after 2 years. In the article Hyperoptic claims to have reached to 750k homes, with CityFibre reaching 1+ million homes, Community Fibre 435k homes and G.Network passes 300k+ premises. So adding all these you get to 2.485m premises. 30% of that is 745.5k or 3/4 of a million. Of course that could be over estimating as 2 years haven't passed on all served premises but then some areas might have >30% penetration rate. There are plenty of other altnets but my rough guess is that this doesn't add more than 1/4 million more customers. So my rough guess is that altnets have about 1 million broadband customers.
Now compare this with the Top 10 UK ISPs By Subscriber Size:
https://www.ispreview.co.uk/review/top10.php
It's good to see Hyperoptic in the top 10 already. I will expect Community Fibre to soon overtake KCOM which only serves Hull. But as you can see the altnets have limited impact in the market due to the reduced network reach. The big telcos also use the offers on their dual/triple/quad play bundles to keep their customers as some people want to have a TV subscription and bundling up can save you money if you need/use all the services.
I am on the VM Ultimate Volt bundle paying £ 79/month. If you take £ 45 away from that to discount the 1gb internet that leaves £24 for all these services: 230+ channels including Sky Cinema, BT Sports 4K, Sky Sports HD, unlimited telephone calls, Netflix Standard and an O2 unlimited SIM with free roaming. The SIM plan alone or Sky Sports are each worth more than the remaining £ 24!
So the big telcos are competitive but only on their top bundles which might not suit everyone.