Order local ISPs to block the site, forbid companies and/or individuals with local presence from doing business with the site, or I dunno, maybe require everyone in the country to install an MITM cert like some backwater autocracy. Not really my problem how you want to run your own country. No country should have the right to enforce laws globally.
replyMost UK domestic ISP's are part of the IWF (Internet Watch Foundation) where your net passes through a transparent squid proxy. [0]
replyAs well as "family friendly filters" that block rouge sites via their own DNS.
[0] https://www.blocked.org.uk/about
And if you're a person of interest your connection is routes through GCHQ Cheltenham.
So it wouldn't be to hard for Ofcom to apply a filter. If they did expect domestic ISPs will comply but hopefully independent ISPs won't.