Do you tip the waiters in the Concorde room? The cloak room attendant? The cleaner in the toilet? Or even the agents at the front desk (though I have seen USD 100 bills inside ticket jackets).
I had a discussion about the spa with a senior BA manager at LHR and his point was that the job was well paid with good work benefits including set breaks so tipping wasn't expected as at a commercial spa. I've had dozens of massages in the past 2 years and have never tipped -- the service is simply too short. There is little turnover of staff so they know be my sight and the lack of tipping hasn't worsened the service. But I do sense that they have come to expect tips over time. FWIW the best massage I have ever had at LHR wasn't in the MB spa, it was in the arrivals lounge and was pure luck (I arrived just before closing time and was the last client for the day, thus got the best treatment). Twice as good as the MB spa gives.