Once upon a time there were arrestable offences and non-arrestable offences. Certain acts, such as violent assault would guarantee a trip to the cells even if you were never charged.
Now, since SOCA, all offences are arrestable.
This means that even if a Constable has no prospect of a conviction, he can still arrest you, transport to Police station, detain you for 28 days (rare but possible) and then release you without charge.
You have no recourse, no right to compensation for what ever loses you incurred. You have effectively been tried, convicted and sentenced by the arresting officer – even if he holds you for only a few hours. Although this maybe an extreme example, cases are rife where Constables, being human, have abused their power of arrest simply to prove a point about who holds the power.