The BKS days at LBA were good, but never 12 a day to LHR, unless you are doing the Ryanair trick of counting the inbound and outbound - there might have been 6 return flights at some stage but most of the time it was 4 or 5.
The memory of BEA Viscounts was that BKS used them for a couple of rotations a day for perhaps a year before they got their own Viscounts. The Heathrow based BEA pilots were very wary of flying into Leeds and diverted if there was anything approaching low cloud or a cross wind