There is currently no legal requirement for UK ISPs to enable content filtering however all the major ISPs have agreed to voluntarily implement this. Whilst in theory users have an option to 'opt out' in practice most users are unaware of this and the process varies considerably depending on ISP.

As far as court ordered blocking, IIRC only the top 5 ISPs are currently required to implement this. Not sure what impact this has had on subscription rates for smaller providers.