Queuing at Heathrow can be hours long. If I drop off there, I never wait there, you don't know how many people are ahead of you in the queue.
Best to come back straight into town.
At Gatwick, the police are targeting Uber drivers from waiting there for a job, as it's not in the PCO area, you can't wait there to receive a job (though you can if you're in the PCO area). Again, best to head straight back in.