I think not.
Capacity is there. And more is coming with T5C.
Incorrect, almost all the gates at T5C are already in use with coaching to and from the stands. When T5C opens it will just eliminate a lot of coaching. It will barely increase capacity at all.
Are you aware that BA operate to a number of destinations from T3 because there is not enough space in T5? It has long been known that T5 would never be big enough able to handle the entire BA operation. An unfortunate consequence of the space constraints at LHR.
One proposal in the late 80s was for T 3 and T5 to be fully linked by a large cut and cover trench with underground people movers and backup walkways as well as transfer baggage belts and backup roadway for tug drawn trolleys so that in effect they were one terminal with 2 entrances,- one in Central area (T3) and the other in T5. One day something of that sort will happen but it would have bee cheaper and easier to do at the outset.
There is a lot of talk of extending the T5 transit system from T5C over to T3 - seems sensible with oneworld connections etc.
The combination of the extra distance of T5 from London, and the longer time taken getting through the terminal because all destinations are mixed together, must have made a considerable hit in BA's domestic carryings. We have several in our office who say that transferring to BMI on the Scotland routes now saves them half an hour or more each way to/from London.
Oh come on, extra distance from London? All of about 2 minutes on the Hex. BA's domestic services are much more punctual than they used to be from T1, and more punctual than bmi's too. If it is such a big issue why are bmi downgrading domestic routes like ABZ to ERJs?
You are also ignoring the fact that a substantial proportion of BA's domestic pax are connecting onwards. Don't think they're going to be complaining that (virtually) all destinations are under one roof.