Actually, even people from quite modest backgrounds had some of their clothes made for them.
For example, if you read The Road to Nab End by William Woodruff, you will see that his first suit, when he was 14, was made for him by the Co-op, who were considered better value than other tailors because they offered two fittings, not one. Woodruff's parents worked in a cotton mill in Lancashire, and their fortunes fluctuated with the economy. when Woodruff was 14, their fortunes were definitely on the way down, but the money was found to get him a decent suit. Novels of the period are also full of references to men in badly-fitting ready-made suits, showing that this was something to be remarked on. according to my mother, her three elder brothers, born between 1923 and 1927, all had their school shorts made for them. Their father worked for the Inland Revenue.
One of the points about having your suits made for you is that they would last a long time. My father has one made about 1980 which he is still wearing. A few months ago, having discovered that the same tailor was about to retire, he rushed out and got a tweed jacket made!
However, a lot of other clothes would be made at home. There are frequent references in novels to young men and boys wearing pyjamas made by their mothers. My mother certainly made dresses for me, and for herself, in the 1960s. My maternal grandmother, was, however, not at all domesticated, and couldn't cook, let alone sew! She did play the piano to concert standard, however.
Ann