Section VII of SIS
The British Resistance organisation
The Secret Intelligence Service (SIS, aka MI6) began planning a nationwide British Resistance, to operated after any Nazi occupation in Spring 1940. This was deeply secret, not least because SIS were not supposed to operate within the UK (this being the province of MI5). Even within SIS its existence was concealed, managed under the cover of the SIS accountancy section (Section VII) by long-time SIS oficer David Boyle. Boyle was a lifelong SIS officer, although no hint of this is given in his autobiography. Those that were recruited to this shadowy organisation were given only the minimum information needed to carry out their individual tasks, never knowing who exactly tgave their orders beyond vague references to 'British Intelligence', and were ordered never to speak of their work. Most never did. This, rather than the GHQ Auxiliary Units, was the real British Resistance organisation.
The existing SIS network in Ireland was used as one model for the organisation, comprising a number of cells, some of which were linked by wireless. At the time this was a novel structure. The primary function was to collect intelligence, which could be transmitted to any government in exile. There were also a number of sabotage cells (whose arms caches can be dated to as late as 1944) and also 'lone wolves' who would carry out individual missions .
To protect the organisation, its members were ordered not to become involved in opposing the actual invasion. They would only be activated once the country was actually occupied. For this reason, men and women who were in reserved occupation or were too old to be called up were selected (their age having the unfortunate consequence that most died before feeling able in more recent times to reveal their secret). Those already in the Home Guard were instructed to keep a low profile. Teenagers were also used as couriers (sometimes using roles as ARP Messengers as cover) and, in at least one case, were trained as wireless operators. The agents would then serve as a cadre to expand the resistance if and when necessary. To that end it is highly likely that SIS saw the training of saboteurs at the Osterley Home Guard training school as a useful reservoir of potential recruits. The focus on making home-made incendiaries, explosives and timing devices was certainly in line with the methodology of Section D (and in sharp contrast to the training of the Auxiliary Units). There are only tantalizing glimpses of the existence of the Resistance but this was a nationwide organisation with cells documented in Norfolk, Suffolk, Sussex, Isle of Wight, Somerset, Cornwall and Devon. Some training was carried out at the Altcar training range on Merseyside. Oral testimony includes other cells in Worcester, Birmingham, Nottingham and Manchester. There is anecdotal evidence for arms caches in Snowdonia and Birmingham and archaeological evidence for a mid-war arms cache from Mellor, near Stockport. The puzzling report of a cache of 30 SMLE rifles and Enfield revolvers found in the roof space of a former Manor House in Grassthorpe, Nottinghamshire in 1993 is potentially another example of a section VII cache (mistakenly identified at the time as related to the Auxiliary Units but now being researched by CART).
The existence of the deep cover Section VII Resistance was protected by what, at first sight seems a confusing muddle of other organisations that had guerrilla or intelligence-gathering function. The ruthless hope was that while the Gestapo were tracking down members of the Auxiliary Units, Section D Home Defence Scheme or the more unofficial Home Guard guerrilla units, they would not realise that there was a hidden layer of British resistance. Indeed, the organisation of the Special Duties Branch of the Auxiliary Units seems almost designed to be sacrificial and quickly broken by an enemy. This layered methodology was explicitly promoted by SIS in countries about to be occupied, always stressing that any guerrilla activity, assumed to have a very short span of survival, should have no contact with the longer term resistance organisation.
Britain was not invaded by the Nazis but part of the agreement to allow SIS to operate in Britain was that Section VII would assist MI5 in internal security. Any spying on the British public made the operation particularly sensitive and there was also the possibility, that having created a wartime intelligence network in the country, it might prove useful in the on-coming Cold War! This added to the traditional policy of SIS never to comment on its operations, whether present or past.
The existence of the Section VII Resistance was only revealed (in a single paragraph) in the official history of SIS by Keith Jeffery in 2010 (The History of the Secret Intelligence Service 1909-49). The first detailed account was in Fighting Nazi Occupation: British Resistance 1939-45 in 2015.