We are having a 12 hour layover at Heathrow beginning at 9am. I am trying to decide if we should use the tube to get around to a few things or use a hop on hop off bus. 3 people in the group have never been to London. They want to see globe theater, tower of London, Westminster Abby and Cathedral. My husband and I are very familiar with London and the underground system.

I wouldn't do the H-o-H-o. I'd pick the one or two sites they want to see and just visit those. With a 9AM arrival -- after immigration and leaving your carry-ons at Left Luggage you might get into central London by 11:30-ish. If your flight out is at 9PM you would need to be back on the tube by 6 or 6:30 PM so you'd have enough time for a nice lunch and visit a couple of sites. I would not bother with the Tower UNLESS you actually visit. And getting there at mid-day the lines will be enormous.

You could take the tube to Victoria (change to District Line at Hammersmith), walk to/visit Westminster Cathedral, see the Palace and walk through St James's Park, visit Westminster Abbey, and up Whitehall to Trafalgar Square, then St Martin's in the Field and Covent Garden, then tube back to LHR.