You need to take this on board as a warning sign.
Being on the same page financially is very important.
Not being delusionally optimistic us important.
Not shutting down or dismissing your partner by calling her pessimistic for bringing up inconvenient facts is important. It won't fix things. You cannot bury your head in the sand.
I used the HSBC mortgage calculator tool for you. To get a 200k house on 40k combined salary a year, you would be allowed to borrow 190k.
If you borrowed 185k (because you would have a 15k deposit), chose a 25 year mortgage length, your monthly repayments at 3% interest would be around £877 a month. Add council tax and bills and you're looking at a monthly bill of at least £1000k once you add in council tax and internet. Food bills, gas, electricity, transport, flat repairs if it needs any would then take it up to maybe 1300k or more. Private pensions, savings money for medical bills, holiday money would be on top of that.
I chose 3% rate because after the initial fixed rate, you would be on a variable rate, and you repayments rare could go up to 3% or more.
You said he was trying to apply for train driver jobs. Do you know for sure he's actually applied for one or has he simply said he's working on it? As far as I'm aware, becoming a train driver us a pretty rigerous process. Do you think he can cope emotionally if one day someone throws themselves on the tracks?
You need to ask yourself how you would feel if you got a 30k job, and he either changed his mind on being a train driver or simply wasn't successful. What's his plan B, and would he be willing to do that plan B? Please don't be one of the many women who thought their partner would change and step-up once the baby came along.
Maybe you can get a flat for 200k in London, but will it be an area that you want to live in? Will it be safe or family friendly? Have a park nearby for the kids?