I don't think that the 1-day overstay due to a flight issue is itself an issue, but a history of staying for the absolute longest amount of time permissible doesn't help contribute to giving the officer the feeling that your main ties are to the UK! When you give the impression "I'm trying to stay in the US every last day that I legally can", it's not that far a stretch for the officer to feel that you may one day extend that to days that you legally can't. I'd suggest some significantly shorter stays (1-2 weeks) to try to dispell that thinking.