Just thought I would add a couple of points in relation to slots. I'm no expert in this area (and do not have my reference on all this to hand) so others should feel free to correct me:
The slots at most main UK airports belong to Airport Coordination Limited, an independent company whose sole purpose in life is to allocate slots and deal with disputes. The main shareholders in ACL are the main UK airlines IIRC, though they are forced by law to act independently and to follow some fairly strict rules.
Slots are awarded on a season-by-season basis, with (Northern Hemisphere)Summer running from late March too late October, and Winter running from late October to late March. All IATA carriers set their timetables according to this seasonal cycle. At the end of a season, all slots return to the slot pool - even BA's. They are then reallocated for the following season.
What BA, BMI and everyone else DO have are "Grandfather Rights" to particular slots. Each summer, BA and BMI get first refusal of those slots they had last summer. The same happens each winter. This is an IATA principle, not a European one, and means for example that AA and UA are just as entrenched at ORD, and LH at FRA, as BA are at LHR.
The slots are lost if a carrier does not actually use them at least 80% of the time during a season. ACL can also remove them if it looks like the slots are unlikely to be used in a given season (for instance if the carrier ceases trading).
A slot for a given season can be exchanged for another slot with another carrier at the airport (or, as I understand it, at another ACL airport, allowing LHR-LGW swaps). Legally speaking, these swaps have to be done free of charge (the intention was to stop slot trading). Airlines have ignored this prohibition and have been fairly blatantly selling slots, though the selling airline will always receive a pair of duff slots from the buying airline in these cases (in addition to the millions of pounds changing hands). It doesn't help that the main authority with power to stop this process is... ACL.
Airlines will also apply for additional slots, subject to these not being taken by carriers exercising their grandfather rights. Under EU law, new carriers to an airport are first in the queue for any slots left over - this is how we have seen carriers such as Etihad, Jet Airways, China Eastern, East African Safari Air, etc got viable LHR slots over the last few years. If easyjet or Ryanair wanted slots at LHR they could apply for them, but would probably get only a pair or two per day - enough for a symbolic presence only and therefore useless to an LCC.
Slots are therefore only as secure as the carrier holding them. When Air Inter, a French regional carrier, went under a few years ago, their slots were forfeited and then allocated to easyjet and others (after an almighty set of arguments involving the French government and the French version of ACL). Air Inter's creditors did not get a penny for them. When Pan Am went under, by contrast, it had the good sense to sell off its LHR rights and others to carriers like United and Delta.
Slots could also in theory get reallocated if the allocation process were to be changed. I'm sure that BA and others would seek to claim some value to their Grandfather Rights (and I am also sure that such a claim would have some sort of value to it) but I'm not sure they could actually prevent such a change outright. Of course this won't happen at LHR (or elsewhere in Europe) as long as other airports around the world have national hubs with the local carrier holding most of the better slots.
Finally, BA (as they continually point out) have a smaller percentage of LHR slots than LH have at FRA or AF at CDG. LHR is also a more popular airport for other long haul carriers. I would say BA have more competition than most, especially on long haul.