For I-751: You do not need to expain anything. It was a temporary trip to visit family. As long as you load the application with evidence (joint finances, joint leases, joint tax return transcripts etc ) - you will be fine. You didn't terminate lease etc before going to the UK, right?
For N-400 under 3 year rule: You will provide all the trip dates etc. Officer may asked whether you travelled with your US spouse or not. Technically, you can be denied by a very picky officer. The likelihood of this is not too high.
You can always file under 5 year rule and it wouldn't be a problem at all.
P.S. I might have misunderstood your post. If you had some marital issues, I strongly advice NOT to file N-400 under 3 year rule. Wait for full 5 years and apply at that point. Of course, you have to go through I-751.