I will tell you something I havent told anyone!!
I was on the valhalla ride with my children.
It is alike a log flume type ride where you sit in a log boat low int he water.
I was 16 stone.. I did feel that at times things were a bit sluggish and I was a bit paranpid about being too heavy..
ANyway Just as we reached the hot steamy section(those who ahve been in the ride will know what i mean) there were a couple of sharp bends... the boat was def lower int he water my side

and on the last bend we got absolutely stuck!!
The boat wasn't moving anywhere... so all other boats on the ride stopped moving too..
Anyway... we all had to get out and walk up this ladder sort of thing.. up the side of the ride to an emergency exit!! It was pitch black with jsut some emergency lights and quite scary as I had young kids with me!
Anyway..t he ride was stopped and the queue dispersed! I left first as felt it was soemthing to do with me although no one said anythign at all!
However.. I heard the ride manager talkign on his walkie talkie to the engineers... I over heard the engineers saying... " You didn;t have a big fatty aboard did you???"
I never told anyone what i heard but am sure that there should have been a weight limit on that ride!!
Also as a side, I was also on avalanche.. i went on with my then 6 yr old... you sit with the person infront between your legs... No one said anything But I actually hurt him!!! On this ride you are very thrown about and the pressue meant that at times he was taking my full weight pressing down on him as well as a bit of gravity... He was crying when he came off and I thoughthe had jsut been scared by the ride... I was horrified later when bathing him... he was brusied all down his ribs and his hip!!
Soooo the answer in my opinion is that rides in this country are not made to carry larger people.. its different in the US, but over here they are stingy!
As others have said, it does come down to body shape, as well as body size, I struggled at 16 stone and would never have manged if I had been bigger..
I am not trying to rain on your parade though just passing on my experiences... I love theme parks with a passion and this is one of my goals for when I am slim, to make sure I do as many as I can in the coming year!!!
Have fun whatever you decide!! If you do wait till september you will be several stones lighter and wont have to contend with any queues either so it will be a win win situation!!
xx