In choosing a title for this poem, Rupert Brooke, feeling homesick, had a few titles in mind but eventually settled on The Old Vicarage, Grantchester. During his years at university, this was where he stayed.
Brooke longs for the simplicity of Grantchester as you "may lie / Day long and watch the Cambridge sky." His references to Tennyson allude to his education and "The Men who Understand" (with the purposeful capitals he uses) are his peers, indicating his university education. He casts aspersions on most other towns in the district, stressing the purity of Grantchester where men "observe the Rules of Thought." Again, his capitalization stresses the higher education status of the men from Grantchester.
A university education was held dear and this could even be a reference to education - "the holy land." He asks if the chestnut trees are still there where, presumably he lay "in reverend dream" before he qualified - "the yet unacademic stream" referring to those who are studying now.
The water metaphor, the stream, the river and "the water sweet and cool" is a fairly typical image of life as older persons discuss how their university years were the best years, without the stresses of modern living and realities were more like "dreams" and there was "honey still for tea."