With things as they are, you need a completely different mindset to afford children. If you really want them , then it's worth making all sorts of adjustments. Some people would describe these as 'sacrifices' or 'not having a life' so you need to decide whether having children or being able to spend money as you please is the more important. Your financial situation will hopefully improve in the future, so would you you regret not making the effort to adjust in the shorter term?
From what you said, you have about £2300 'left' for food/clothes/utility bills/insurance/travel. It's tight but if you make 'saving up forMat Leave/Nursery your mission on in life, then you can do it. Especially if you get the rest of your family on board. You don't need to buy expensive presents for other adults – you could just send or make a card, or even write them an appreciative letter, or if you must have a 'thing' then some nice soap/supermarket bunch of flowers/small chocs.
Is it possible to get family on side about this and explain you need to cut back and stash some money away for the Mat Leave/Nursrery Days. Hopefully they will be happy to not be 'treated' as often by you,if this is the reason. But if you don't tell them,they can't be on side.
Even if you conceive tomorrow, you will not need nursery for at least 10months and hopefully longer, which gives you a bit of time to stash some cash. Treat every penny you save as making your family dream possible.
Visiting family: if you feel you have to treat them, instead of taking them out for lunch, take the food with you. Much cheaper and can be as much or aslittle extra work as you can bear.
e.g not much extra work: go to the deli and buy cooked meat,supermarket for bagged salads +other veg for making salads inc potato or pasta salad, buy a dessert or cake, take a bottle of wine of a soft drink e.g.elderflower cordial which you all like
or some extra work: buy a joint or a ham/chicken and cook it yourself to eat cold, make a cake/trifle/fruit pie or crumble/make salads and side dishes/buy french stick& chop up & spread with garlic paste & oil for cheaper garlic bread
Could you ask them to provide the food when you visit and you just take a cake?
Could they contribute to your petrol cost for some of the visits?
Can you host your family?: you said you have a spare room, so unless some particular problem e.g. with mobility/toileting needs, you maybe could. Even if the room isn't as posh as you'd like, if there's a bed and bedding it will suffice. They are coming to see you,not to be in a hotel.
Your home: you said you have a spare room. Although you will need this eventually for the child,probably not for a couple of years so you could get a lodger in the meantime. Could your wfh desks go in the box room,or at least one of them and the other in the living room?
Saving on energy costs: get a slow cooker. They are worth their weight in gold for cost saving and convenience, esp when you have kids or if you and your partner are working odd hours and not always eating at the same time. Are you leaving things on standby/lights on unnecessarily?It's only pennies, but it adds up. Could you take shorter showers?
Saving on casual spending: if money is really tight then eating out/coffees out/takeaways/beauty treatments/presents for friends & extended family/soft play & farm parks are just not possible (except maybe very occasionally) Same with impulse magazine buys (you can borrow magazines from many libraries) and any sort of impulse buy really. Do you really need new clothes or can you be imaginative with what you already own, e.g. use different scarves/jewellery or other accessories. Could you swap clothes with friends? All these are cheaper even than buying second hand, but this is also a good option as long as you stick to a budget.
Christmas/birthdays/Mothers' Day etc: we have all been conned into spending lots of money for these and we just don't need to. Especially if it ends up as you give your Aunty a box of chocs and she gives you a box of toffees/or if you feel there is a certain amount you have to spend. Actually getting together with family & friends is more valuable than just spending money.If you have to get a gift,have a budget limit/make them a cake/write a card or letter with good thoughts in it/offer a service e.g babysitting for family/making dinner at their house/cleaning/help with gardening/lending them tools.
When you have your children: find cheap or free things to do. Walking around your neighbourhood/ parks/watching anything 'happening'e.g. building site/trains going under bridges/anyone working (although ask to make sure it's OK)/postman emptying the postbox/shops getting deliveries esp barrels arriving at pubs/ for slightly older children noticing things in your neighbourhood e.g door knockers, counting lamp posts when on a walk/collecting stones, twigs, conkers/blowing dandelion clocks. You get the idea! You ca get books from the library with loads more ideas.
Saving on toys: buy second hand but also use what's in your house: cardboard boxes for dens/hiding/make into tunnels/ egg boxes and the packaging just for handling (& using as craft materials)/playing at the sink with a sieve, spoon, whisk, yoghurt pots for pouring.
Of course books from library. And singing – you don't have to be good your child isn'tamusic critic!Imaginative games: I used to set out an obstacle course of newspaper on floor as steping stones, a cahir to climb over, a stol or table to crawl under. This is cheaper than soft play.
Just a bus journey esp if it'sa necessary journey is endlessly interesting to small children, if you point out things they can see, let them hand over the money etc.