Oops Nats!
Yeah, its illegal Emmy, although they'd find a way round it. However, they won't have to pay your maternity pay so they're not missing out financially. I was pregnant when I had a job interview and didnt tell them. They were fine about it when I fessed up. Heavy lifting while pregnant is fine, as long as you're sensible about it. I did it all the way through my pregnancy, as I had quite a physical job. If you don't feel comfortable doing it though, you'll have to either tell them or make an excuse up.