Are you absoultely sure you could afford a mortgage at £1,300 a month? This would leave you just £540 to cover your pension, council tax, food, bills, car etc. Tight even with interest at the lowest level for 400 years.
And what if interest rates climb? For every 1% increase in interest you would lose another £180 off your £540 . If interest rates got anywhere near their long term average of 8% at any stage you would be absolutely stuffed.
Of course you could fix for five years at the end of which you would have to either hope rates are no higher or that you've had a good pay rise. Don't forget you'd be spending that five year period or longer living on £540 a month - not something I'd chose.
All in all there needs to be a cap on income multiples to manage these unforeseen risks that your forgetting / ignoring - interest rates, changes in circumstances (jobs, family), increased costs (inflation) etc. A cap will not remove the risks but ensure people are more likely to be able to cope if and when they occur.