Most UK domestic ISP's are part of the IWF (Internet Watch Foundation) where your net passes through a transparent squid proxy. [0]

As 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.