To have borrowed £300000 the borrowers would need an income of £75000, unless they thought it was all easy, interest rates would stay low forever and talked the lender into letting them borrow more than 4x salary. Stupid if so.
A monthly salary, take home, on £75000 is circa £5000. So, new mortgage payments are less than 30% of income.
In 1989, my girlfriend and I were paying over 15% interest and over 50% of our salary went on the mortgage.
Nothing to see here.