I'm on a brief break from work at the moment, so can't do a lot of digging, but -
resigning without a job to go to is rarely a good idea - for one thing you're generally not able to sign on for JSA, it also takes some explaining to next employer.
especially if there's a way of taking a step back (even if that means taking a day or two's holiday or sick leave) to think about it all and / or seek advice.
employers have a duty of care when it comes to preventing / dealing with sexual harrassment in the workplace.
if you're in a union, they should be able to help (may need to refer to branch / regional office or something, especially if the other person is a union member as well.)
ACAS have a page about it
here.
This suggests that it comes under the equality act. some things under the equality act don't have the 2 year qualifying period for unfair dismissal (constructive dismissal is a form of unfair dismissal) and getting sacked / victimised for raising a complaint under the equality act may also be covered. (i'm not clear on this, so i'm suggesting it as a possibility, not with any authority)
to claim constructive dismissal, you usually have to show that you have at least tried to resolve a situation via employer's grievance procedure before you resign.
hope you get things sorted one way or another.