Well, I've got several issues with some of the NCL experience, but security isn't one of them. Yes it can be a bit of a wait in the corrals at peak times, but that's partly a perception based on being able to see all those people in front. I always find the security staff personable, tolerant of irregular travellers, and willing to inject some humour into what is inevitably a hassled part of the airport experience.
As to using the East Coast to transfer to Heathrow, I have now given up using the train on the East coast Main Line for anywhere en-route to London. My experience is that the punctuality is appalling, and when things go wrong, it quickly escalates into a major disruption. Couple that with the bewildering frequency of problems on the Underground, it would make a connection with a LH ex H'row a very stressful experience.