Makes sense. Should’ve figured.
That’s Atoz-speak for “That makes sense. I should have figured that out myself.” 
That is a slightly misleading description of their relationship. Morse is Thursday’s “bagman” which translates as something closer to personal assistant. It is part of his job to ferry the senior officer around. It would have been a sought after position for a junior officer as a good career move and caused grumblings in the series when Morse was given the post ahead of more experienced officers.
Writing dialogue in screenplays has to tread a fine line between realism and comprehensibility. Jargon is normal in almost any trade and cops have as much of it as anyone. The writer(s) would bear in mind their intended audience. Morse and its prequel Endeavour were both written for a British (largely English) audience.
At least, Oxford-based policemen would not generally have had a strong regional accent, but I have no doubt that many “real” conversations between colleagues in the police at that time (and, probably, now) would be pretty incomprehensible to anyone not well versed in “cop-speak”.
As a Detective Inspector, Thursday would not be expected to drive himself. As Qmatic says above, it would have been a desirable position and normally given to a sergeant.
The Oxford working-class accent is west country-lite with a strong London-english influence. A traditional accent in that part of the wrold I would guess would be closer to Pam Ayers’ accent as she comes from a village only 15 miles from Oxford.
Colloquial speech in a work of fiction isn’t really subject to “definitive proof”: its meaning can be so heavily dependent on context, character and the interaction/relationship depicted. I think it’s misleading to think of this as “slang” in the same way as rhyming slang, back slang or variations in regional dialect. It’s just a speech habit between colleagues.
I suppose someone might have done a detailed study somewhere on this particular part of colloquial speech, but I have no idea how to find it.
The term ‘much in’ isn’t a specific idiom or phrase though. It’s just a very terse question that would make perfect and clear sense in the context presented.
I’m just resurrecting this to say thanks to @Machine_Elf for prompting me through this thread to watch Endeavour. I watched the original Morse series when it first aired, and I was vaguely aware that they had made this, but I didn’t know how good it was. I can’t remember a show I have enjoyed more in 10 years or more. It’s a superb production, and since I was born in southern England in the '60s, a lot of the cultural context is especially enjoyable.
I don’t know if you’re aware of the series Lewis - post-Morse, where Lewis is no longer the sidekick. It’s different, but good in some of the same ways.
Also if you like Morse, Endeavour, etc. You might like A Touch of Frost
Thanks - yes, I’ve seen A Touch of Frost; Lewis is next on my list, although I may first rewatch the original Morse series, I haven’t seen it in a long time.
Nostalgia is part of the appeal for me as well. My mom was from England, and I visited there several times in the '70s. The cars, clothing, and behaviors in the later seasons of the series are not so terribly different from what I remember back then.