It is not a simple first come first serve at LHR. I work for BA, we frequently wait whilst Saudi or AA or other airlines jump in front of us. Especially when another heavy has just departed. It's not about who arrived at the RW first, it's all about how they can get the maximum aircraft airborne as quickly as possible.
As for vectors, poor knowledge on your part. Firstly you don't know whether they slowed up very early - something that does NOT affect your place in the queue... With ATC permission if you come back to min clean after you enter UK airspace with a big delay then you don't lose your place in the queue. It could be for various other reasons too. If you want to see blatant priority then there are many places I could suggest, LHR is not one of them.