esudfu,
Heathrow seems to be a bit rigid in their visit system, but Birmingham were very easy going, so it depends on unit.
I'm afraid we haven't got a visit system at Heathrow. The lady whose details are given out by HR hosts visits in her 'spare' time, despite the fact that she is actually one of the busiest people on the unit...she ends up working late and starting early to make up on the time she spends hosting the visit. To host a visit at Heathrow takes a considerable portion of someone's work day, given that we have to sort out airside passes with BAA, and then come airside to meet the visitor, collecting them and bringing them to the tower (and the same when the visitor leaves).
Understandably, there are only certain days when this can happen, given other work commitments.