It was not always so flexible. During/after the Oxford movement, some clergy went to prison for stretching things too far (which was one of the driving forces for prayer book reform in the 20th century).
Remember that "low church" Anglican might still have meant clergy in cassock, surplice, academic hood and preaching scarf (and in the cathedrals, copes); a surprising amount of dressing up for a Baptist, I would have thought?
There's a wealth of information out there. As it happens, at the moment I'm reading a book called "The Rise and Fall of the Incomparable Liturgy," which gets into all of that in great detail, if you want to read more.
As far as preaching goes, Gordon's right that it depends on the style of the preacher, but as a general rule, I'd say that more evangelical churches expect longer sermons with more detailed exposition of Biblical text; more Anglo-catholic churches expect shorter sermons, and are more willing to engage in reflections which are less Biblically-based. (I've preached across the breadth of the spectrum, so I've had some experience of both).