If I show up after consensual sex and say I was raped that man has to be arrested
Recorded figures for male against female rape for 2013:
15,670 crimes recorded by the police.
Out of which:
3,850 'detections'.
2,910 got to court.
1,070 resulted in convictions.
Which means that only 6% of recorded rapes in 2013, got a conviction.
The actual number of rapes over the year (based on home office crime data) is estimated to be in the region 78,000. That is not many rapists banged up for their crimes, even taking rapists of multiple victims into account.
So, no, I'm afraid that if you think that you can say you were raped, the police rush round and arrest someone , and then prison is a forgone conclusion, you are very sadly mistaken.