I think that two big problems are that a) English (I am leaving Scotland and Wales out of this as I don't know about their curricula) schools don't devote enough time to history and b) England has no set, "content-specific" curriculum for history.
Yes, there is a curruculum which sets out aims and suggests topics. But there is no curriculum which sets out "Year 1, autumn term; Study the Ancient Britons, covering X, Y and Z". Rather, schools create their own curricula which vary widely. I've looked at the primary schools in my hometown-some schools don't have a discrete history subject at all, just "topic work" which supposedly includes some history. Most do have some sort of discrete "history"-but it is so different from school to school, and varies from year to year. Then you get pupils from a bunch of different primary schools going to different secondary schools whch also don't have a single standardized history curriculum. You can easily see how repetitions and gaps occur---how pupils could wind up "doing" the Mayans twice or WWII three times, but study nothing on the Anglo Saxans or the civil war or the British Empire or the modern slave trade or.....
There is a good article called "The strange death of history teaching" which I read a few years ago. As the author pointed out, it's persistently claimed that traditioanl history teaching approaches did not teach the Empire, the slave trade, Ireland, women/gender and other areas, but it's simply not true. The following are given as just a few samples of the kind of questions that were constantly set on exam papers in the 1950s and 1960s.
Give an account of the Revolution settlement either in Scotland or in Ireland. (Oxford and Cambridge Schools Examination Board, GCE O level, Paper II English Outlines, 1399-1714, July 1951, Q 15).
‘Sugar, spices and slaves were the basis of the first British Empire.’ Discuss. (Oxford and Cambridge Schools Examination Board, GCE A level, II English History Outlines, 878-1939, July 1965, Q 28).
Account for the growth of the movement either for the abolition of slavery or the improvement of working conditions, and explain their success. (Oxford and Cambridge Schools Examination Board, GCE A
level, II English History Outlines, 878-1939, July 1966, Q 34).
Discuss the economic consequences of the extension since 1914 in the employment opportunities for women. (AEB, British Economic and Social History, 1969, Advanced Level, June Paper II 1851-1960,
Q 10).
Illustrate and account for the progress made before 1919 towards the emancipation of women. (Oxford and Cambridge Schools Examination Board, GCE A level, II English History Outlines, 878-1919,
June 1953, Q 36).
In what respects did the treatment of the poor change between 1750 and 1850? Do you consider the general position had improved or deteriorated? (AEB, British Economic and Social History, Advanced
Level, June 1969, Paper I, 1750-1850, Q 2).
Teach MORE history, spend more time on the subject---that is what we need to do.
I think history and similar subjects get sidelined due to the pressure to spend almost all curriculum time on maths and English (which are seen as being the "serious" subjects, while history is often seen as a bit of larking about in the afternoon).
I live overseas and take care of my daughter's English education at home, and I take the approach of "devote much of your time to teaching science and history and geography BUT in ways that involve loads of reading and writing, so that the child learns English literacy AND also learns lots of content and knowledge at the same time." I love this approach and my daughter knows a lot of history (UK and global) even though she has grown up overseas and the amount of time we have available for English-language instruction is a fraction of what the average British child can enjoy.