I know this is a rotary forum, but I hope you'll let a plank post...
The "BAA" Slots are indeed the ones which if you abuse will get you a ban. They are adminstrated by a company called Airport Coordination Ltd (
www.acl-uk.org) which is owned by a selection of the world's favourite (and debt-ridden) airlines.
For the majority of GA (i.e. non-airline) customers a slot has to be obtained through a recognised handling agent (BA Exec or Metro for the majority of us).
The times that ACL use are those provided by the Stand Allocations Unit (or HOC as it's mainly known) - I'm not entirely certain where those numbers come from - probably Air Traffic.
If your delay is ATC inflicted (Brussels ATC slots for us fixed wingers) then that is an acceptable delay from ACL's point of view and is allowed for in the Adhoc operators procedures (on ACL's website). I don't think that it allows for delays caused by third party ATC delays (i.e. the post above about an aircraft landing at LTN following a slot delay and then a subsequent helo trip across would not be a valid reason for being late on a slot).
It's all related to Heathrow's limited parking space, not runway capacity. LHR have said that they can increase movements on the runways many times in the past (probably before Swanwick decided to send them all off to EGJJ though...), however Business Jets are not allowed to land without a slot even though until 12 months ago there was a GA apron with loads more space than aircraft - difficult to accept when the slots are to protect that apron space!! Can understand it for airliners who need a stand each.
So this shouldn't be a pop against ATC. They are a really good bunch, who will happily fire us locals off from the south runway even if it's in arrivals mode. If they can't get you in without a bit of hold then bear with them, they are busy after all. But the ban isn't anything to do with them, it's to do with a lack of education of GA and Helo operators. If you want to know more go and look at the Adhoc procedures on ACL's site.