Unfortunately I agree with @daisychain01 and her advice from a couple of months ago, she’s hit the nail on the head.
Contrary to popular belief, you can’t just resign because you don’t like something about your job and then bring a constructive dismissal claim. To succeed, you have to legally demonstrate that a) your employer committed a fundamental breach of contract, b) you resigned in direct response to that beach and c) that you did so without delay.
You have problems on all three points here. Regarding a) the actions of one person don’t necessarily amount to a fundamental breach of your employment contract. You’d have to raise a grievance and go through the correct formal channels (because arguably, how could your employer ever address a situation they were unaware of? What if they’d decided to sack the perpetrator as a result of a grievance process?). On b) you haven’t resigned in response to any alleged breach. You resigned because you had another job. And on c) the delay point isn’t relevant if b isn’t proved, but if it was, you didn’t resign without delay. Usually you would resign without notice - not continue to work for a few months afterwards, because it completely erodes any argument you would have had that their conduct was so serious that you had to leave their employment. By carrying on without raising a grievance, you’ve given them the argument that you’ve accepted and waived their behaviour.
My suggestion would be to raise a grievance and highlight his behaviour (ask for their grievance procedure, they are legally required to have one). At least then he will hopefully have to face some consequences. But I’d consider carefully whether it’s worth bringing a claim. Most of your compensation would be based on loss of earnings and if you have another job to go to then it wipes out that compensation element completely.
If you could link his comments directly to breastfeeding (which on the face of it, aren’t directly related) then you could potentially claim for that, however you also have to bear in mind that there is a 3 month limitation period so unless you acted quickly, or could demonstrate an ongoing course of harassment related to sex, then you’ll be time barred from bringing that claim.
I appreciate that this post seems negative but it’s also realistic - CD claims are very difficult to prove because the burden of proof is on you in the first instance. I’d only embark on one if the facts and timeline fully supported all of the three points I mentioned above.