One aircraft at a time? Definitely not. The separation will be increased for the reason given by BOAC but never to the extent of one at a time. The Air Controller in the tower determines the spacing and various factors are taken into account. 6 miles is a good starter and this maybe increased if pilots have trouble leaving the runway or a turn off is closed, etc.
Many major airfields have surface radar which will show any obstructions on the runway. It is ATC responsibility to ensure that the runway is clear.