The Law

In the UK it is legal to look at porn so long as it does not feature: under 18s, sex with animals, torture, scenes of rape or sexual assault or violent scenes which are life threatening or likely to cause serious harm.
Porn that is made, and made available, in the US has to be certified that all persons were over 18 at the time it was made (known as 2257 information).
Porn can be shown on UK TV after 9pm so long as it does not show erect penises or close ups of genitals.
The legal age is 18 to buy porn magazines or videos: most porn websites try to prevent under 18s from accessing them, either through charging with a credit card or by a disclaimer on the front page.