I was born in 1963, so I was a child/teenager in the 70s.
My home diet was pretty dire, home cooked most of the time, but cooked very plainly, vegetables boiled to within an inch of their lives, meat with no seasonings except bisto or an oxo cube. Liver once a week, boiled fish once a week, mince as a cottage pie or mince and onion pie, leftover roast meat as a very bland curry or cold with a salad, sausage and mash, egg and chips. Never had chicken.
We had a roast on Sunday, overcooked beef, nice roasties and yorkshires though. The oven only really went on once a week on Sundays for the roast, so a sponge cake or some buns would be made then as well, to last the whole week.
Maybe twice a year we'd get fish and chips. Now and again we'd have fish fingers, or Findus crispy pancakes, which I loved. Puddings were non existent except an apple pie on sunday, which was spoiled by putting cloves in it.
I think people were thinner in the 70s because very often the food was just fuel, it wasn't nice to eat, there wasn't much incentive to overeat.
Edited to add I loved the school dinners, ate every scrap, even the ones other kids grumped about. So I think my home food was particularly bad