A colleague of mine always uses a valet parking service at LHR, not sure if it is the same one. He always speaks highly of it.
I have to be honest and say that I just don’t trust them to drive my car or where they park them. Particularly after the problems at LGW which were on the TV a few years ago.
Many years ago the only long term parking at LTN was valet. I remember collecting my car on the one and only occasion to watch the guy move the car infront of mine as he reved the guts out the diesel engined car infront of mine. It took me days to get the seat right!