You've had some good advice on here OP particularly Dungeon's post.
Only spend in cash - it's easier to not think about it when it's just a swipe of a card.
Get anything you need second hand as the first option. Many second hand things are in great condition (possibly because of the "keep buying new stuff" mentality many seem to have!) - particularly baby/kid stuff which is expensive in the first place and usually not used for long.
Have a direct debit going out at the start of the month for savings, don't leave it till the end of the month as more than likely you won't have much left. Put some savings into a stocks and shares ISA for long term savings.
Pay in the maximum allowed in your workplace pension scheme so you get the maximum employers contribution. Open a SIPP and put a little bit in there each month - even just 10 or 20 quid will add up over the long term.
Go back to work when possible even if PT after baby is born, to keep on the career ladder if nothing else.
Don't buy stuff. A simple one! If you think you need something, wait a bit and see if you still need it in a week/month.
Get rid of any expensive subscriptions like Sky etc - if you still want access to tv get a cheaper one like Netflix etc.
Plan ahead. Do a weekly/fortnightly meal plan before you go shopping and stick to what you need for the plan. Also plan ahead for things like lunches at work, coffee etc - make and take your own.
Prioritise free/cheap activities - e.g. visit to local community farm rather than lunch out.
We lived on one £14k salary for a bit at one point (no benefits or the like), and just didn't spend loads on going out/buying stuff, but still had a good life - i don't remember it being all that different to how we live now to be honest.