I think there are many larger mortgages around. £400,000 feels normal to me: all the decent houses where I live are at least this much.
I have a friend in London who has a mortgage for around £540,000 (for a flat). I'm well outside of London (east anglia) and my mortgage is £300,000 but this feels cheap- I had to move far out of town to find a house this price (3-bed semi). I have a friend in Manchester with a mortgage of around £400,000 (4 bed semi).
We're all FTB in our late 30s (before people mention 'starter homes') with 10% deposits, and bought in the last year or two, paying a large amount of our incomes to the mortgage.
I was paying a similar amount in rent per month to the cost of my mortgage, so it didn't feel that expensive. Before people start going on about all the "additional" costs of owning, in the last year I've paid out £140 (plumber and locksmith), so that argument feels like bollocks.
At the same time, interest rates have rocketed since I bought, so my monthly payment would be more if I had to borrow now: I pay around £1400 per month, but that might be more like £1900 per month at today's rates (I haven't done the maths recently, so I might be a bit out).