How can the prologue of The Remains of the Day be contextualised from a historical or cultural point of view?
I know that it starts July 1956, the date of the nationalisation of the Suez Canal, which heralds the end of Britain's long reign as the world's foremost colonial power. Also, Stevens' reluctance to embark on "what might have seemed a presumptuous speech" is an exemplification of the famed British "self-restraint", pointing to the English butler as an icon in the popular imagination.
The context of the prologue is indeed important because, as you suggest, it marked the end of the Brtish Empire with the Suez Crisis. In Stevens' mind, however, the glory days of the Empire are for him the best days of his own life and he talks with affection about 'tradition' and times when 'the greatest ladies and gentlemen of the land gathered'. It is not until later in the novel that we realise that some of these gentlemen were in fact Nazi sympathisers and actually the period Stevens thinks of so fondly was a period in which shameful things occurred.
You are right to point to Stevens' self-restraint and reluctance to talk to his new master which contrasts sharply with his master's more casual ways such as his love of 'bantering'. Stevens even plans to wear his old master's suits for his journey to see Miss Kenton, a clear statement of his longing for the past.
The fact that the novel is written in first person is also clear from the Prologue and we get a clear insight into Stevens' mind although we do begin to question what really happened as we read on. He does not seem to feel he has a voice even though he has been in charge of a large household. Clearly Mr Farraday represents the new 'American' way of doing things and is not always familiar with what is 'commonly done'. Stevens is signalling the beginnings of the influence of American culture on Britain which will continue with the advent of rock and roll, and so on.