Other things to bear in mind as they do have an effect on costs. You won't be buying lunch every day or commuting / travel costs to work. But as you say, you'll be in the house more - heating / lighting. You will probably go out less so that's a potential saving.
I don't think babies clothes are particularly expensive, you'll get lots of pressies. Nappies will be about £10 per week, formula if you're not breastfeeding might be another £5-£10 per week. If yoou are breastfeeding, your grocery shop might go up because you'll eat for England! Once a baby is on solids, jars can be expensive but if you just puree up your own meals, there is not much additional cost. A pram can be anything up to about £1000 if you go for new, top of the range, but have a look on your local FB selling site or ebay and you'll be surprised how much you can save by going second hand. The same for a cot, moses basket, changing table etc.
I agree that nursery can vary massively depending where you live. DD's 3 day a week place was £819 per month 2 years ago, would probably be more now (we're in the North West). I obviously lost two fifths of my salary on top of that. However, your employer (or your DH's) may offer vouchers or a salary sacrifice scheme which means you give up part of your gross salary for childcare vouchers / payments. Means you don't pay the tax and NI on that part of your salary. Agree that depending on what you do / how part timers are viewed, you need to think carefully about the impact a prolonged maternity leave / going part time will affect your long term career prospects.
Wrap around care here is about £20 a day (breakfast club and after school club) and bear in mind that most children will probably do some activities (e.g. swimming lessons, football maybe). Of course, none of that is compulsory but it is expensive (swimming about £8-10 a lesson, we pay £34 a month for DS's football). All of that is a long way off though. Also if have regular holidays, once you are stuck with school holidays, the cost will most likely triple. Holiday childcare here is about £30 for a full school/work day (per child).
Also think about things like pension contributions - if you (or your employer) contributes say 5% of your salary, your contributions obviously go down if you go part time.
Other couple of points to bear in mind - some mortgage providers won't consider your full salary whilst you are on maternity leave (even if you are going back full time) so just think about timings. Also think about how you will manage on the income you'd get whilst on maternity leave (i.e. can you save now?). Also think about increasing life insurance once you have dependants to consider how much you'd need to cover mortgage / how you or your DH would live / work / manage childcare if (god forbid) something were to happen to one of you.