Like any wildlife, they are where they are, but Bushey isn't huge, and I usually park up by the Diana fountain and start wandering from there.
You quite often see them as you drive in, just don't do what I did a couple of years ago, concentrating on one particular stag, I didn't realise another one had appeared behind me, and I had to take some rather swift evasive action rather than be in between them. It's very easy to get engrossed in the one you are shooting and not realise what else is going on. I normally go with my wife these days and she acts as lookout