It's all relative @EllaSW. Even at 35% net income, which is a big chunk, you're still at the upper end.
Using examples again:
35% of £5,000 = £1,750 leaving you with £3,250 for everything else
35% of £15,000 = £5,250, leaving you with £9,750 for everything else
Your lifestyle and committed outgoings will determine whether nearly £10k a month is enough wriggle room to service a £1m mortgage.
Bigger house will be more expensive regarding bills, maintenance and upkeep, but the real consideration will be if you have any DC, and what you spend on them. I had two DC at nursery together for 2 years, and although I could afford it, seeing that £2,500 go out by DD on the 1st of every month made me wince. Even though I was still able to put a further £2,000 into savings every month.
If you have 2 DC in private schools, the costs will likely be similar/more, except you're committed for much much longer. You'll also have to think about how you'd pay for big ticket items such as a new car and holidays, which can be thousands all in one go. If you sit down and write down all your average expenditure for each month, you should be able to work how comfortable comfortable is.