Plus, Dark Angels have that...y'know... secret thing going on.
Went to the FLGS this evening and hooked up with a couple of local BFG players. I told them of the meager fleets I had managed to put together and it looks like some capital ship combat is on the honizon. Yay!
Now...enough about me, back to the topic at hand.
I'm thinking ThePov is correct: Dark Angels wouldn't relinquish command willingly to an IN fleet, admiral, grand admiral, or any other rank you want to pull out. The only Imperial service I could see them "submitting" to would be the Inquisition...and even then I'm pretty sure it would be a careful dance of diplomacy and threats to get the DA to go along with it (especially when you add in the whole Fallen Angels business).
So with that in mind, I'd say SM BB with MotF is the way to go if you're wanting to stick to your fluff (which I'm pretty sure you've already decided you're going to do).
Wow, I typed all that and just now realized I could have typed in, "What he said," and said pretty muich the same thing. Sheesh.