The poem "Ambulances" by Philip Larkin is a bit dismal and depressing. The theme of death and finality is woven throughout the lines of this poem. It is free verse, so although the lines do not rhyme, they do convey a story of all the lives that have passed on via ambulances. The ambulance itself symbolizes the "angel of death," carrying the passenger's body on to the next life.
For the stanza you've specified, I will break down each line with a summary underneath.
For borne away in deadened air
This line indicates that the body is being "carried" (borne) away, transported into the next life. The air (oxygen) is no longer alive in the body's lungs. It is dead.
May go the sudden shut of loss
The idea here is that life is fleeting; a person may be here one minute but gone the next. It may be sudden in the sense of time, despite a person being ill. In one instant a person may be breathing and then stop. A heart may be beating and then stop. If the person in the ambulance is close to death, then their death could occur during the transportation.
Round something nearly at an end
As an ambulance is driving along, rounding corners to its destination, the person may be approaching death, rounding out their life. The person's life is coming to an end, much like the trip.
And what cohered in it across
As the person's life is coming to an end, so are the invisible threads that are unite (cohere) them to something or someone else.
The years, the unique random blend
In the whirlwind of the transport, it is evident that the person is losing their battle. The person's age is now going to be a defining end. The years, the wrinkles (if existent), the scars—whatever makes a person unique—are trophies of a life this person once endured.
Of families and fashions, there
The clothes this person is wearing could perhaps be hand-me-downs from family members. Even if the clothes are not borrowed, they still represent the fashion that is either trendy and current or forsaken with time. It reveals the person's tastes, preferences, and interests, giving readers a glimpse into this life that is coming to an end.