This is totally at the discretion of the Heathrow Number 1 Director (The Master Director if it is 'split' No.1 N and No. 1 South). The vectoring to the landing runway includes both right and left-hand circuits and if spare capacity is identified in the opposite circuit then an aircraft may, subject to mutual agreement, be taken 'over the top'. It is often an option if there are 'too many off the stack'. The only constraint is that the aircraft must remain within the joint radar vectoring area. The Number 2 (Final) Director isn't bothered which side they appear from as long as they are sensibly spaced for the base turn. Certain ATCOs favour its use more than others when under pressure. Colloquially known as 'Over the top' meaning over the top of the Final Approach sequence hence the initial descent from minimum stack level to (say) only 5000'. It happens occasionally off BNN too then they are taken left-hand. The Radar Directors are mentally 'counting' the track distance all the time for 'Continuous Descent Approaches' and the 'Over the Top' is yet another path-stretching 'tool in the bag' along with 'orbits' and 'once more around the hold'. It can also happen on easterlies as well but much more rarely as there is greater room to maneuver in the first place.