When you are brought into a police station under arrest, the custody officer must tell you about three key rights, both out loud and in writing, and these rights continue throughout your detention.
First, the right to free and independent legal advice. You can speak to a solicitor in private, at any time, day or night, and it costs you nothing. You can change your mind later and ask for a solicitor even if you declined at first.
Second, the right to have someone told that you have been arrested. You can have a friend, relative or another person who is likely to take an interest informed of where you are.
Third, the right to consult the PACE Codes of Practice themselves — the rule book the police must follow.
You should also be given written information about the reason for your detention, your right to regular reviews of that detention, and how you will be treated. If you are deaf, do not speak English well, or are under 18 or otherwise vulnerable, extra safeguards apply (such as an interpreter or an "appropriate adult").
This is a plain-English summary. For the exact rules, read the official PACE Code C.