I think the OP probably meant that she was close to departure and needed the answer quickly to understand what to do at LHR, not that time was tight when she arrived.

Now obviously by now she will have arrived, but for future reference for anyone who has the same query currently the only terminals that handle domestic (and Republic of Ireland) arrivals are T2 and T5. From NCL, it'll obviously be a BA flight into T5.

Here, one of two things will happen, meaning it's always a relatively quick exit - either the plane will get a jetbridge in T5A, or a bus gate which will drop the passengers right by the domestic channel, eliminating any walking.