What do British police say when they arrest someone?

In the UK, when police arrest a suspect, they must tell the person that what crimes they are suspected of committing that has lead to the arrest and they will then issue the arrest caution:

"You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something which you later rely on in court."

The exact Wording does not need to be used. Once the arrest caution is issued, the issuing officer must ensure the suspect understands.