I think Channel 4 have snapped up the exclusive rights 😉
If the box sets do appear, I guess they'll be within the 'abc' channel section...
Yes you're right, there is a lot of crossover between sky box sets and NOW TV. For MAOS, it's available for Buy & Keep on the Sky Store boxsets, not the regular box sets- this suggests to me that Sky (and by extension NOW TV) dont have the on demand rights for this show, they can sell the DVDs but not make the content available for streaming.
For anyone that's still interested NOW TV have now added the S1-4 boxsets of Marvel's Agents of SHIELD.
Funnily enough I had S4 recorded from E4 and never got round to starting until a couple of days ago, I'll be able to carry on with no ads on NOW TV now!