An endowment was the only mortgage we could get, others were available but not available if that makes sense. Building societies had a limited fund for mortgages and demand outstripped supply. We had just enough for a deposit which we transferred between 3 different bldg societies over a 3 month period based on 'rumours' that said building society had funds to lend.
A mate introduced me to a financial advisor who for a fee of (I think £150, maybe a bit more) told us to put our money into Leeds Permanent in Southall and a week later we had a mortgage, low cost endowment but that's all we could get if we wanted a mortgage.
Two years later when we sold the house at an almost 50% profit (irrelevant because our 2nd house had increased by the same percentage), we were able to switch to a repayment mortgage, I cant remember what payment we got off the the endowment, it was minimal but irrelevant in the great scheme of things given the increase in property value.
That would have been end of 1977 or early 1978.