What is a "terrace house"?
UK TERRACE HOUSE CIRCA 1942

What is a Regimental Headquarter Company? What about usage? "Headquarter" or "Headquarters?
Both.... The Regimental Headquarters (RHQ) is the heart of the Regiment. It is where the Commanding officer and his HQ staff control the Regiment, assisted by the various departments of Headquarter Company.

and .... this platoon would be attached to Headquarter Company and so more immediately under the C.O.'s authority.

Where are Chawleigh and Chulmleigh?
Just Google 'em right up.

Also, DEVONSHIRE REGIMENT

~S~