we go to Chessington's all the time (live just around the corner)
we only go on the smaller rides at the moment, as dd2 is too small to go on much (and would protest a LOT if we took dd1 on some of the bigger rides ) so can only help up to a point.
I have got the exit pass info for Chessington's somewhere - will have a hut and see, and post later if I find it. I think you need something in writing sating diagnosis, and the idea is that your ds has no concept of queuing - so "just" (don't mean to belittle at all - this is thier view) using a wheelchair for eg isn't enough.
we haven't actually registered for exit passes there as we go so often (have annual passes) that we can pop in for an hour at the end of the day, when queues are tailing off, and since we are only doing the little rides there isn't a huge queue anyway.
the Sealife bit is quite good, but gets busy, crowded and noisy - echoes a lot in there.
the zoo is ok, but can be hard to see some of the animals - easier to see form the sky train, but does have stair entry/exit (quite a lot of stairs on the exit)
the dds (4 and 2) love: (all these are fairly easy entry/exit
the carousel
the flying elephant ride
the dragon boat ride (a bit tame really, but the only water bit we can all go on)
Mr Toads cars (ime less of a queue than the Tiny Truckers, for practically the same ride)
dd1 likes the Bash Street bus (dd2 too small!)
The Ferris wheel is ok, but can't see a lot really.
bubbleowrks is fab (bt has strobe lghting in case that is an issue)
I can't advise on any other rides, as dd2 just won't grow! [grin (86cm now, at 2.6 - can't wait for the extra 4cm, which will open up a whole range of rides for her)
let me know if you want any other specific info