Consider this....
The relationship between LBA and MAN and LPL and MAN is quite different. Analysis of the peak month in 2008, shows that 75% of LBA's passengers were carried on routes where the airline / tour operator also offered capacity ex-MAN. Just 7% of LBA's traffic was on routes not available from MAN in any capacity.
By contrast, 75% of LPL's passengers were carried on routes where the airline WASN'T offering capacity to the same destination ex-MAN and 33% of their passengers were on routes not available from MAN at all.
In short, the macro view is that there is generally appetite to compliment services between LBA and MAN. A further insight is that the ratio of LBA scheduled passenger volumes to MAN volumes on routes offered from both airports is about 1:3 - i.e. a route handling 60,000 passengers a month ex-MAN generates 20,000 ex-LBA.
So, how is all of this relevant to the 'London' debate? Well, as pointed out by many, the bmi route was primarily used by connecting traffic, with c.70% of passengers connecting at LHR. It's demise leaves KLM's service into AMS as the only genuine interlining service remaining from the airport and in its current form that delivers about 10,000 passengers a month connecting through AMS. In comparison, MAN handles about 250,000 passengers a month that are connecting through the various hubs it serves.
Using the 1:3 rule a credible case could be argued therefore that there is a market for connecting traffic to/from LBA of about 80,000 a month - somewhat higher that than 10,000 on offer by KLM. Not unrealistic given that when SN were on the LBA-BRU route and LHR was 5 x daily F100s, the total connecting market was 35,000 a month. That was in 2000 and the air market in the North is now about a third bigger.
So given the sizeable opportunity and the complimentary nature of the MAN /LBA markets, where is it going wrong? In short, it is the location of the hubs. They exist in either congested European centres like LHR, CDG or FRA where slots are difficult or in faraway places in the US or the Middle East . In some cases they are just as congested as Europe but in all cases they present restrictions on which aircraft can offer direct services from LBA.
In summary, it is hard to argue against the fact there is something to get after at LBA regarding the connecting market - a prize which is big enough that the existence or otherwise of the LHR service is only partly relevant. The LBA management team do have experience of overcoming all of the challenges of slot restrictions and short runways as a quick scan down the BRS and NCL departure boards reveals and they already have a feather in their cap at LBA regarding PIA, so who knows........