BA have always wanted their own terminal at LHR to facilitate transfers. They wanted to build it themselves, as has been done by other carriers at other airports around the world.
When T4 was built they wanted it for themselves but were refused, although they have about 90% of it. I would not be surprised if the do drop LGW but the completion of T5 is some way off and who knows what state the biz will be in by then.
On the transfer thing, as I live near to LHR I have only been in transit through it once. When working in Germany and on a trip to South Africa where the best price was with VS. But because it was with two carriers, it was two separate legs and no chance for the baggage to be lost.
On Monday, I am going to South Africa again but this time via Germany! Crazy I know but I am using the LH miles that I earnt whilst working there. Now both legs are with LH, so I wait and see. The last time I did a similar thing with LH, my bag missed the connection. That was MUC/FRA/DTW but it was hand delivered the next day.