Originally Posted by corporate-wage-slave

I would be just about OK about this since you are HBO. Assuming an on-time train arrival, and allowing a generous 10 minutes to get down to the depths of the Jubilee line (and it will be fairly quiet at that hour) you should be in Canning Town for 06:35 and thus LCY for 06:50, and hopefully before. Gates close at 10 minutes to departure, some gates do have a physical door that they shut too. So that is all plausible, but there is about 5 to 10 minutes slack in this. You can ask the front person in security to use the near side security channel - used by crew - if it's looking tight (so after 06:55 hrs).

But you say this isn't your usual operating mode, and so I wonder if you really want to start your day like this (whereas it's pretty normal for me). So why not look to book a hotel - perhaps using points or even Avios? Some of the hotels in the frame here aren't expensive either, cheaper than a cab I imagine. So there are hotels like the two Travelodges (one very close to the airport), Hampton, DoubleTree, Holiday Inn Express, Premier Inn (that's a noisy one though), Aloft - all within walking distance of the airport. Several hotels by Canning Town station would take almost the same time to get to the airport, and slightly further off there are now many options in Canary Wharf and Stratford, which would give you a more interesting evening the night before departure.

It is best to at least put the day of the week in questions like this, since Monday and Thursday are different from Tuesday and Wednesday.