In the days before the National Curriculum and Ofsted, there must have been huge variation in what was taught. Somebody upthread said she never studied History at all, which I find astonishing. I thought everybody educated in UK schools would have had some history lessons at some point, although only a minority would have gone on to study History to O level/CSE/GCSE. At my school both History and Geography were compulsory O levels, which I think was pretty unusual.
As far as I can remember, in what you'd now call years 7-10 we cantered through Roman Britain, raced through the Anglo-Saxons and Vikings, dwelt a bit longer on the Norman Conquest and the centuries that followed, lingered on the Tudors and Stuarts, and then leapfrogged over the political and military history of the early Georgian era entirely. We went straight on to the Agricultural and Industrial Revolutions, with a brief reference to the American War of Independence. For O level we finally studied something other than British history - World History from 1870 to the present was the syllabus. We covered British political history in the late 19th century including the battle for trade unionism, wider suffrage and the Suffragettes, and the divisions over Irish independence/self-determination; the Balkan conflicts within the Austro-Hungarian Empire and the causes of World War I; the Russian Revolution; possibly a bit about the Boer Wars (it was nearly 50 years ago - the details are a bit hazy now).
Slavery would have been mentioned in passing, I'm sure, but the triangular trade was not a topic we spent any time on at all. I didn't hear that term till several decades later. There was probably more emphasis on Wilberforce and the abolition of slavery across the Empire, i.e. the bits that showed the UK up in a better light. At that age I think I was more aware of slavery as an issue in US politics than in the UK. We watched the TV mini-series Roots, which I found horrifying.