I think they could word it better. The only clue it might only be for business accounts is the title and the link.
https://business.zen.co.uk/internet-access/ip-addresses but the text that makes thing ambiguous are as follows:
"A single, static IPv4 address is available free of charge with all of Zen's broadband services. However, we know that in some circumstances you may require more, which is why we offer blocks of 8 to 1024 IPv4 address from as little as £0.83 (ex VAT) per address, per month."
Which to me, sounds like there isn't a restriction. But, given the only page I could see this info on was a page about their business product. It could well be that's now a limitation.
What I find interesting is that when they were going to remove the original free /29 from the legacy product earlier this year there was an implication that there was a paid product for extra IPs. But it seems that maybe that is only for business accounts.
As it turned out, they let me keep a /29 (except it was moved) when I queried it. But if they put their foot down, seems there's no real way to keep it with Zen any more.