Abuse of position for a sexual purpose is treated by the police inspectorate and the IOPC as serious corruption, not as a conduct matter at the margins. It most often involves someone the officer met through their work who was vulnerable at the time.
Where to report it
You do not have to report this to the force the officer serves in. A complaint about a serious matter of this kind can be referred to the Independent Office for Police Conduct, and forces are themselves under a duty to refer the most serious matters to the IOPC whether or not anyone complains.
If what happened may have been a criminal offence, that is a separate route and it can be pursued at the same time. Section 26 of the Criminal Justice and Courts Act 2015 created an offence of corrupt or other improper exercise of police powers and privileges, which covers an officer exercising their powers for a sexual purpose.
Why this page carries no claim prompt
The circumstances vary enormously and the right route depends heavily on the facts, including whether criminal proceedings are on foot and whether the person affected was a child at the time. This page exists so the routes are findable. It does not attempt to tell you which one applies to you, and anyone in this position should get individual advice rather than act on a general page.
How long you have to bring a claim
Publishing a general figure here would be worse than publishing nothing, because a reader who relies on the wrong one loses their claim outright. Get your own position checked, and do it early rather than late.
The free routes, before any claim
A complaint to the force costs nothing and does not stop you claiming later. If the force's answer does not stand up you can apply for a review, and across all forces reporting, 24.5% of completed reviews found the force's own outcome was not reasonable and proportionate.
The three routes, and what each one gets you.
Start with your force
Complaint handling, review outcomes and stop and search practice vary enormously between forces. Every force has its own page here with its own figures.