I think the MB treatment and arrivals lounge at LHR is only available for BA J & F pax - they give out invitation cards in flight for both. In any case, the MB is high in demand and if elligible, should be booked as soon as you arrive (shower area on lower level) - no harm in trying. Likewise, for the showers, get in early. The evenings have high shower demand and you will wait at least 30 minutes (you get a beeper to tell you when your shower is free) - at least you can attack the bar whilst waiting. If lucky, you'll get the shower with full water jets and eucalyptus steam etc - no water conservation problems in this area!

The BA FT Board has many thread on the Arrivals Lounge and the MB.