Lilac,
This is going to be a bit blunt. I thought twice about posting this, but I think it's best to be honest.
I think you are being unrealistic if you think that a health and safety assessment will entitle you to changes to your working hours in your situation. I have read elsewhere that your daughter is now 12 months old, and also that your doctor thinks that breastfeeding is important for your mental health.
The HSE rules are about removing health risks associated with breastfeeding - exposure to chemicals, etc. They also recommend good practice around allowing breastfeeding mothers to rest, express in suitable rooms, store milk, etc. They aren't really about changing your work to allow for a physical presence to do all the feeds you might want to.
What you are essentially saying is that continuing to breastfeed is particularly important to you because of your mental health. I think you need to go back to your doctor and explore whether your PND is now a disability within discrimination law. There sounds like a good chance it is. If it is, your employer will be obliged to make 'reasonable adjustments' for that disability, and issues like travel would be part of that. TBH, I am surprised if your doctor hasn't raised this.
If your employer is on the ball, they may have twigged the disability angle from what has already happened, but they may not. I think you are going about this the wrong way.
I am sorry, I know that must be disappointing after you have waited so long for a response on the health and safety side.