Hi Jack!
No worries, I hope I can help you although I am not a ground agent and can only share my experience (and what my passengers report to me) with you.
North terminal's busiest time is in the morning when all the US/Caribbeans and Middle East flights depart as well as the normal european sheduled and charter ones. Very early in the morning there are less people covering security as well so that might lead to longer queues. I believe that, being a tuesday as opposed to a saturday when MANY charter ski flights depart at around 0600-0700, 90minutes should be fine for your flight. If by any chance you see that you're running out of time for checking your bags DO approach any of the BA ground staff. Your check-in/baggage drop off will most likely be in zone F, where all the european/mediterranean flights are dealt with.
Once you're airside it might take up to 10-15 minutes to reach some of the gates (101-110) and about the same to reach gate 46. It takes a bit less to reach gates 54-51 or 58-63 but they are very often occupied by the american flights/carriers. If you depart from gate 45 (bus bays) be careful as it can be quite confusing down there at times, we have gate 45 A, B, C etc!! They once put 3 geneva flights (one BA scheduled, one BA charter and one GB charter) at the same gate leaving more or less at the same time (very well thought!!
) and that created a lot of confusion!!!
Make sure that nobody from your party gets lost in the terminal (quite good shopping there) and remember that no flights are announced in the terminal anymore, so do keep an eye on the screen!!
I hope this helps and if you are in any doubt, please do approach any of the BA ground staff, they are usually very pleasant
Have a great holiday!!