For each page, I have added the respective custom domain and got Cloudflare to add the DNS record automatically. Each page has the DNS record:
Type: CNAME
Name: keelecompsci.uk (or respective custom domain)
Target: keelecompsci-uk.pages.dev (or respective dev URL)
Proxy: On
TTL: Auto
I also get the warning “This hostname is not covered by a certificate.” on each DNS record however this wasn’t a problem in January 2023 when I set up these sites. I haven’t touched these pages in 4 months since I set them up at the start of this year. I have recently needed to check on one and found it was down with all of my other pages.
There is, as @Laudian has noted, an issue with the registration of all three domains @billonium4535. This information (which I initially overlooked) is available in whois <domain>.
There is also information to this effect when a lookup is done via the Nominet website as per screenshot below.
Looking up clientHold on the ICANN website it says
This status code tells your domain’s registry to not activate your domain in the DNS and as a consequence, it will not resolve. It is an uncommon status that is usually enacted during legal disputes, non-payment, or when your domain is subject to deletion.
This explain why none of the domains resolve.
As per the information shared by @Laudian, there is an issue in verifying the registrants (your) information. I suggest you review your information with the registrar to ensure it correct.