I also can't see how your house buying figures add up. You say his flat is worth £300,000 and he will have 80k equity when he sells it. Presumably you are counting on his current mortgage being portable, for which you usually have to buy within 6 months again. Or would he start a new mortgage, which will probably come with a term of 20 or 25 years, based on his retiring at 65. Will he lie to the mortgage provider and tell them this is his aim?
To buy a £600,000 house on this basis, with 80k deposit between you, you will have to convince a mortgage company, in the present lending climate, to lend you £520,000. Based on the standard 2 1/2 times your salary, you will need to be earning £208,000 each, and be in secure jobs. Or you could be earning £100,000 and he could be earning £308,000...or vice versa...
I didn't know teachers earned that much, but perhaps you earn hundreds of thousands of pounds per year?
You will have to get a joint mortgage if buying a property together. Its not just a matter of saying his current flat is worth £300,000 and he will get a £300,000 mortgage just like that, with an 80k deposit, and you getting 3 or more times your salary on top of that. For joint mortgages, its usually 2 1/2 times the joint salaries. Even if it wern't you would need to be earning around £70,000 pa to fund that extra £200,000.
Anyway, once you have your £520,000 mortgage, you will have to pay it. I think this will cost about £3000 a month. Once he retires at 55, you will have 10 years of paying that on your own, and house prices and salaries might not rise as much as people used to think were a guarantee. You will also be saving (presumably) to send a child or children through university...