I know with GDPR you can’t publicly see the registrant of a .co.uk domain on Nominet Whois. Can it be done any other way?
No because Nominet WHOIS is the master WHOIS. Meaning that the record comes from Nominet. It can be replicated anywhere else, but you can't just get information that simply isn't there. But you can do other things. For example, sometimes when I want to check on my competitors registered domains I an run a reverse nameserver query with <insert-API-here>, if they have unique nameservers << !important That'll return all of the domains under that nameserver (and hence, all the domains they own)
If last Registrant change is before GDPR then contact details are probably accessible, but still against the rules.
I did a reverse nameserver lookup to find other domains on the same ip. I also got help from an AD member. Both yielded results, thanks everyone.