There is a recommendation that we should have no more than six aircraft on conditional clearances, however if there is no chance whatsover of any misunderstanding, I don't see any reason why there couldn't be one or two more. The danger of lining too many up is that when an aircraft arrives at the holding point with a tight CTOT, then it can be potentially more difficult to revise the departure order. I usually indicate their projected number in the sequence to aircraft when it is busy. The number of aircraft with conditional clearances is largely academic; having identified the traffic they are following, pilots only have to remember one aircraft and then line up behind it. If there is any uncertainty then they only have to request confirmation of the order. If there are several aircraft of the same company/type then the controller will identify the aircraft to be followed more precisely.
If HF was able to successfully identify the aircraft he was following (as he obviously was), then I suggest there was not an issue.
During single runway operations with one aircraft to depart in the gap, we will only issue conditional line up instructions subject to one aircraft. In LVPs, aircraft will be given individual line up clearances.
In mixed mode it depends entirely on the size of the gaps between the inbound aircraft. Working on a rough 3 nm per minute time separation, controllers can calculate how many aircraft will be able to depart safely in the gap.
The departure sequence is all about safety and expedition; if Heathrow controllers lost the ability to use conditional clearances then capacity would doubtless suffer.
Lastly on the subject of RTF phraseology HD makes a good point. If pilots were to read back the clearance exactly as passed, then controllers would be a lot happier!! In the example quoted it would be, "After (not behind), the departing BM B737 on the North side of the runway, line up and wait 09R". The condition comes first, the description second and the instruction last.
My biggest bugbear is with pilots' readback of runway crossing clearances. PLEASE read them back in the format passed as above. NOT, "Cross 09R after the departing/landing (company/type)"!!!!! Easy isn't it?