On the stamps : the 1d red is plate 175 and the 2d blue is plate 14.

Cancelled with a London Inland Circle Duplex type issued 1872.

I have not yet found information on the person signing the letter, nor on the receiver.  Information on the history of the bank is on Google.

The person mentionned, Earl of Huntington, might be, seen the date :

Francis Power Plantagenet Hastings,14th Earl of Huntington who lived from 1841 until 1885.

There is much confusion and guessing about the triangles.

Here I only consider the "point upward" marks.

Here is a trustable text I have found recently :

"These triangular obliterations began in 1895 to be used for bulk cancelling of "imitation" typewritten circulars (at a lower postal rate). Originally they were singular handstamps. But at the beginning of the reign of Edward VII machine cancellation was introduced and the c.d.s. (circular date stamp-note P.D.B.) usually found in such cancellations was replaced by a triangle."  (M.R. Hewlett, Wiltshire, 1971, "picton's priced catalogue of british pictorial postcards and postmarks 1894-1939)

There are lots of small islands around the coast of the UK and some of these have their own stamps. Most of them are cinderella's (unofficial stamps without postal value). As far as I know there is only one island that uses official stamps for postage to the mainland only, and that is the island of Lundy.  More on Lundy later.