Just ask. No point pussy footing around. If the company has a shit policy or they choose not to hire you for any potential future pregnancies - then the trash has taken itself out and you had a lucky escape.
Employers typically provide a brief overview of the benefits package during yhe interview, but I personally wouldn't dig any further than what they initially share. If they offer you a job, you'll receive the detailed policies along with your offer letter, and at that point, you can decide whether to accept, decline, or ask for more questions on the benefits package offered.
@Sarah thats a really good point I didnt realise it would all be detailed in the offer package. They did just say "good maternity benefits" as he was going through it.
You'll be eligible for government maternity pay regardless so I wouldn't mention anything at all. Yes it's discrimination if they choose not to hire you because you wish to try for a baby in the future but from their perspective they'll pin it down to hiring someone with no desire to get pregnant and therefore they won't start a job and then take a year off etc