If it's bang on 90 minutes, what sometimes happens is that either the HEL flight gets retimed by 5 minutes or the same for NCL, and thus triggers a rebooking process. Then it may swing back to the original timings. Swapping A319s and A320s/321s seems to have that effect. As KARFA indicates, on the day just stay close to your BA App and it will rebook if necessary, however I think the rebooking trigger is 75 minutes for T3 to T5. There again you can slip below 75 minutes briefly, find yourself rebooked, and then the flight ends up on time, but there is no perfect answer there. Anyway, I'd be happy with that connection if the sky won't fall in if you end up going the next day.