Totally literally, security services deal with threats to national security, so terrorism, certain types of organised crime, they work predominantly as intelligence services (hence MI5)
Police deal with more 'regular' criminals, up to low level uk predominant organised crime. Civilian offenders rather than state enemies.
There is often crossover though not least because regular crime is used to raise funds for terrorism and global organised crime syndicates eg drug dealing, prostitution/human trafficking, Jewel smuggling...
Not that surprising they've not yet connected /Dave and scar face Andy as all we know is they both served in Helmand province around the same time. Doesn't necessarily mean they served with the same unit for there to be easily recognisable links. Camp Bastion has up to 12000 troops at any one time, these operate on a rotation I know the army operates on a roughly 6 monthly rotation cycle. And it's not everyone out and new ones in every 6 months, there's variations due up operational requirements elsewhere, personal factors (army will send individuals home for compassionate reasons eg bereavement of a parent, and of course sick & injured soldiers are sent home and these of course are replaced), so it's pretty much a constant flow of personnel. Plus in this case that means cross referencing 10,000's of personnel over who knows what time period? There's around 75,000 full time serving army personnel, plus another 30,000 'part timers' (territorial army). So that's 100,000 personnel records to go through to cross reference.
GCHQ is actually a location but has become shorthand for mi6 as that's where there admin and training is done. Let's just say it's no coincidence it's not too far from Salisbury, and given what's been happening there recently I don't think I need say more 😉. The C supposedly stands for communication, which is kinda ironic given much of the job is about preventing or disrupting communication.
I don't think rogue, I do think possibly patsy.
Puntastic refs on tonight's this week.