The only information I have is this... https://www.ispreview.co.uk/review/top10.php
Top 10 UK ISPs By Subscriber Size

BT (PlusNet, EE) (Est.) 9,300,000
Sky Broadband (UK) (Est. 2023) 6,200,000
Virgin Media (VMO2) 5,653,800
TalkTalk (on-net) 4,200,000
Vodafone UK 1,158,000
Shell Energy 595,000
Glide (Mostly Students) 400,000
Hyperoptic 250,000
Zen Internet 175,000
KCOM 139,700

The problem is that this is only statistics for the top 10 ISPs, it doesn't give information for all of the ISPs or the bottom by lowest subscriber size. It is definitely something I too am curious to know!

If it's any help in your breakdown, Plusnet claims about 1.4 million customers.

Theres also ISP not listed on this site for some reason.