My family's in the resident but non-dom boat given the nature of my parent's work (generally offshore). Basically, you can avoid paying British taxes on your income despite permanent residency as long as you're not considered domiciled in the UK, aren't a British citizen and don't remit the income to the UK. You're *probably* considered non-domiciled if you spend more than 90 days out of the country each fiscal year. But, this isn't a cast iron rule. Effectively, they have to believe that your residency in Britain isn't complete, even if it is permanent. It's rare they reject a 90+ claim though.
It is quite difficult to achieve, however, and the acquisition of this status has become more discretionary after recent court rulings. Also, your claiming maternity leave would probably complicate this, even if you did fall into the 90+ group.