My post is very crass. It's all about the money but that is what you are worrying about. If he buys a house while his partner ( you) owns a house, he will have to pay excruciating Additional Stamp Duty . You need your lender's permission to let your flat out. They are likely to make it conditional on you moving to a Buy To Let mortgage. Rental income is taxable . Capital Gains Tax when you come to sell. Mortgage,insurance,management,repairs and upgrading costs to meet with latest government regulations. A house as an investment is fine as part of a balanced investment portfolio but a headache inducing gamble as the only item. I can understand keeping it if you fear your relationship will not last much longer and you don't expect to inherit £500,000 soon.
It might make more financial sense for you to sell and invest your equity in tax efficient investments/savings/pensions/insurance against a rainy day.
If he buys/mortgages alone and pays other other housing costs -
say... a year housing costs ----------£32,000
Average cost for full time nursery ..............................£15,000.
Costs of hiring a cleaner/mother's help and general baby things ...... £10,000.
Total housing/childcare costs ..................................... £57,000.
A half share of that is £28,500.
He's offering to pay all housing- so more than half. Yes, he will wind up owning a house but he is also contributing more to the house ( including deposit).
If you pay nursery/cleaner/mother's help you will be paying less than half the total ( just over £2,000 per month)and your savings will continue to grow. You won't be paying a mortgage and other home owner related costs. Unmarried, you get to keep your savings and assets, come what may. On the downside, you lose your chance of getting a share of the equity in his house and other assets including pension,he owns.
You will eventually be entitled to some free hours at nursery,which will cut the bill. If your partner's salary less pension contributions is less than £100,000, you will be able to claim £2,000 from government as the "tax free" contribution to nursery costs. You get this as soon as you start paying a nursery and in addition to the free hours.
For your sanity and long term financial security,I'd suggest you get 2 hours a day Mother's Help/Cleaner right from the start and stick to full time work. I suggest some serious cost checking,investment and tax advice,arithmetic, life insurances for you both,( even if it's only till baby is 18) and Wills. You should do this whether or not marriage is likely.
The fact an almost 40 year old's response to baby is to offer you free lodgings rather than marriage is not propitious. If you do split up, I calculate on his current salary the Child Maintenance he would have to pay you would probably cover about half the nursery but none of your housing,mother's help/cleaner costs.That's on the basis he doesn't go for 50/50 care.