I have! I'm entitled to the full NHS maternity leave 'package', but I am the breadwinner of the household and there is a sizeable gap between what I earn and what maternity pay is. I'd like to take the full year, too, which means no earnings at all for the last 3 months...though we may see if I can go back to work after 9 months and have hubby take the last 3 months of my leave...something to see about nearer the time...
So anyway, yes, I would like to hit £15k in savings by the time LO is born, and we'll be starting to try in August/September this year...I'm just under 1/3 of the way there just on my own savings
What I do is give myself a set amount that is what I am allowed to spend for the month after the bills have been paid, and any excess goes straight into my ISA (or other savings account if ISA is full) on payday. I usually find I have some of my 'set amount' left over at the end of the month, and if that is the case, I put that in my ISA too, along with the next month's 'excess', if that makes sense?
We have a joint account for bills etc and then our own current and savings accounts, so it makes this sort of personal budgeting easy to do. Hubby does pretty much the same as me...though he earns quite a bit less than me, I pay a greater proprtion of my earnings into the joint account, so he can still save a decent amount.
We are pretty lucky in that we haven't had to cut down on trips to the cinema, eating out, etc...because we only do that stuff once in a blue moon anyway! So we don't feel like we're missing out in order to save
But I do take lunch in to work with me most days instead of buying it from the canteen, and I limit my spending on clothes and other 'treat' type things to when I think I really need/deserve it...though I'm REALLY limiting my clothes-buying now, as I'm hoping that in a few months time none of them will fit anyway
But yes, I'd really recommend an instant-access cash isa...I just got my yearly interest paid, and it was £94

Admittedly the interest rates are nothing like they were (I once had a rate of 8%, it was lush...), but they're still better than a lot of 'normal' savings accounts, particularly if you can leave a lump of cash in there for a year or more, while adding to it.
I have to admit that when I first looked at the sums, I panicked and thought that we'd never ever be able to afford a baby, but with some careful planning, we will be ok. Money will be tighter, especially with nursery fees once I'm back to work, but I'm hoping our savings will give us a bit of a head start with it all.
Good luck to you, just keep reminding youself what you're saving for if you're tempted to splurge, and you will get there...even a little bit saved regularly will soon turn into a lot