Okay, throwing some figures about. Make of this what you will!
I bought my current house 13 years ago for £201,000. It's now worth £342,000. That is around a 70% increase in growth. Divided by 13 years = say 4% compound growth per year.
If we apply this maths to your house, bought at £400k today, and growth at 4% compounded for the next 35 years, then it would be worth £1.5 million at the end. It will have cost you £670k over that time, with interest added (as per Martin Lewis calculator). So you will have spent £670k, and made £830k profit. And you'll be sitting in a £1.5 million house, with no mortgage or rent to pay for the rest of your lives.
If you stayed paying rent for the next 35 years, at £450 per month cost to you, you will have spent £189k but you will have nothing to show for it. And you will have to continue to pay rent until you die, which could be another 20 years, at a cost of a further £108k.
Interestingly, I bought my first house 35 years ago for £65k. If we apply the 4% compound interest growth model to that, the house would now be worth £262k, which is actually spot on.
All a moot point of course, if you can't afford to find the extra cash for a mortgage right now.
But these calculations do go to show, how something costing you more now, actually pays off in the long run.