As I understand it Northern and Scottish merged with Highland Airways in 1937 to form Scottish Airways which then became part of BEA in 1947.
I am currently reading a book about aviation on the Isle of Wight (The Vectis Connection by Peter Newberry) which includes the following information in a section on Spartan Aircraft Ltd & Spartan Airlines...
"During the 1930s there were about thirty internal airlines in Britain of which some twelve were placed in liquidation. The key to survival meant more robust financing and amalgamation. Whitehall Securities had very substantial stakes in Spartan, United Airways (itself an amalgamate of Highland and Island Airways and Northern and Scottish Airlines) and British Continental Airways. It was estimated that by merging Spartan and United substantial savings could be made, particularly if Hillman Airways could be brought into the fold. This latter company being owned by another finance house, d'Erlangers. Agreement was reached between the parties and a new company was formed on the 30th September 1935 named Allied British Airways. A month later "Allied" was deleted and the company became just plain British Airways with a capital of £245,240. It was agreed that the constituent airlines would retain their individual titles until January 1937 after which they would operate under the banner of British Airways."
Spartan Airlines was also a manufacturer, in this context
the Spartan Cruiser
which was a trimotor (Gypsy Major) monoplane for 2 crew and 8 passenges. There were 2 marks in airline service, the Cruser II with range of 310 miles, max speed 133 MPH (cruising speed 115), and the Cruiser III with a range of 550 miles and sppeds of 135/118 MPH.
Again quoting from 'The Vectis Connection' and relating to service with Spartan Airlines
Cruiser II G-ACSM 'Sussex'
Delivered June 1934. Sent north in August 1936 to operate with Northern and Scottish Airways. This aircraft may have been loaned to United Airways during 1935 (United were the parent company of Northern and Scottish Airlines and Highland and Islands Airways). Impressed into RAF April 1940 but scrapped in July.
Cruiser II G-ACZM
Delivered 13th December 1934. Transferred to British Airways in January 1936 for use on the North of England and Scottish routes. Scrapped April 1942.
Cruiser III G-ACYK
Delivered 16th April 1935. Transferred to British Airways April 1936 but continued to operate Spartan routes until August when transferred to north of England for the Scottish routes. Crashed near Largs 14th January 1938. The cabin section was lifted from the hills by Royal Navy helicopter in July 1973 and taken to the Museum of Flight, East Fortune where it resides today.
Cruiser III G-ADEL
Delivered 18th April 1935. Transferred to British Airways October 1936 for use on the north of England and Scottish Routes. Impressed by the RAF April 1940 but scrapped in July of the same year.
Cruiser II G-ADEM
Delivered 3rd June 1935. Transferred to British Airways April 1936 for use on the North of England and Scottish routes. Hit hangar on take off in fog at Blackpool (Stanley Park) 20th November 1936 - two killed.
de Havilland Dragon G-ACNG
Ex Jersey Airways; transferred to Spartan in June 1935 and to the new British Airways in March 1936 (a paper transfer). It is very likely that this aircraft operated in the livery of 'Railway Air Services'. In January 1937 it was transferred to Northern and Scottish Airlines. Crashed Kirkwall on the 19th April 1940.