You should enter the UK together for a long list of reasons. In your shoes, assuming you are married, take her through the staffed EU entry side of the UK Border, roughly in the middle of T5's hall. As you enter the area put your passport on top so the contract staff managing the queue don't get confused. Her landing card will need to be completed, again hide that until you get to the desk. There are machine entry gates on the right side, but avoid those.

EDITED: Looking at another thread, I think there are a few extra components to this not mentioned above: are you transiting on to NCL? If so, and you are not a smoker, then there's a special UK Border for that connection. Fast Track for this is by Status and Travel Class (keep your previous boarding card), not nationality, enter the fast track channel on the left. There's typically only one or two agents serving that area anyway, so you and your wife can keep together. It goes boarding pass check (BA staff) - passport (UKBF) - photo (HAL) - security (upstairs, HAL) - CCR.