If BA wanted to keep slots warm at Heathrow in the short term, they would simply operate them on existing routes by flying extra frequencies on the likes of MAN/EDI/GLA/ABZ/NCL/BRU/CDG/BHD-LHR for the defined period rather than starting a new route to Leeds Bradford. BA would also not start LBA/LHR in the certain knowledge that they would be pulling off 18 months later to incur a shedload of bad press in the Yorkshire region, questions in Parliament about ringfencing of slots at LHR for regional UK air links etc etc. Why on earth would you?
If they start LBA-LHR, it would be because there is a business reason to do so - e.g. due to the value of new feeder traffic ex LBA which they could get on LHR services, whereas direct competition ex MAN from the likes of Continental, Emirates etc makes MAN/LHR increasingly less valuable as a feeder. If the route gets off the ground, it's because BA see merit in flying it in its own right versus using those LHR slots (today, tomorrow or in 18 months time) to fly something else.
Quite why BMI pulled off LBA-LHR to continue losing money on MAN-LHR - where the average fares were about half those on LBA-LHR - is anybody's guess to begin with.
It will be an interesting one to watch if BA do decide to reinstate LBA/LHR - last flown in 1980, if I recall correctly. I genuinely don't know whether there is any substance or not to these rumours, but the notion of this being a short-term slot sitting exercise is as unrealistic in commercial terms as it would be damaging to the consumer confidence in the route if indeed it does materialise.